Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ 2,142,733	$ 2,204,602	$ 2,131,447
Other comprehensive income (loss) before reclassifications	1,786	(80,500)	1,921
Amounts reclassified from accumulated other comprehensive income (loss)	0	86,710	(34,162)
Total other comprehensive income (loss)	1,786	6,210	(32,241)
Balance at end of period	2,009,188	2,142,733	2,204,602
Net Gains (Losses) on Cash Flow Hedges
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	0	(6,551)	30,717
Other comprehensive income (loss) before reclassifications	0	(80,175)	(2,720)
Amounts reclassified from accumulated other comprehensive income (loss)	0	86,726	(34,548)
Total other comprehensive income (loss)	0	6,551	(37,268)
Balance at end of period	0	0	(6,551)
Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	(2,151)	(1,810)	(6,837)
Other comprehensive income (loss) before reclassifications	1,786	(325)	4,641
Amounts reclassified from accumulated other comprehensive income (loss)	0	(16)	386
Total other comprehensive income (loss)	1,786	(341)	5,027
Balance at end of period	(365)	(2,151)	(1,810)
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	(2,151)	(8,361)	23,880
Balance at end of period	$ (365)	$ (2,151)	$ (8,361)