Derivative Instruments and Hedging Activities (Tables)  | 
3 Months Ended | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Mar. 31, 2018  | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Derivative Instruments And Hedging Activities Disclosure [Abstract] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Gains (Losses) Relating to Derivative Instruments Recorded in Other Comprehensive Income (loss) | 
 The following table summarizes gains (losses) relating to derivative instruments recorded in other comprehensive income (loss) during the three months ended March 31, 2018 and 2017: 
 
 
  | 
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Gains (Losses) Relating to Derivative Instruments Recorded to Income | 
 The following table summarizes gains (losses) relating to derivative instruments recorded to income during the three months ended March 31, 2018 and 2017: 
 
 
 
  | 
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Gains (Losses) Relating to Derivative Instruments Reclassified from Accumulated Other Comprehensive Loss into Income Effective Portion | 
 The following table summarizes gains (losses) relating to derivative instruments reclassified from accumulated other comprehensive loss into income during the three months ended March 31, 2018 and 2017: 
 
 
  | 
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||