Accumulated Other Comprehensive Income (Loss) - Rollforward of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2015			$ 2,895,860
Amounts reclassified from accumulated other comprehensive income (loss)	$ (71)	$ 576	1,741	$ 2,427
Balance as of June 30, 2016	3,116,231		3,116,231
Pension and Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2015			263
Other comprehensive income (loss) before reclassifications			(9)
Amounts reclassified from accumulated other comprehensive income (loss)	(71)	$ 576	1,741	$ 2,427
Change in accumulated other comprehensive income (loss)			1,732
Balance as of June 30, 2016	$ 1,995		$ 1,995