Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ 2,009,188	$ 2,142,733	$ 2,204,602
Other comprehensive income (loss) before reclassifications	1,280	1,786	(80,500)
Amounts reclassified from accumulated other comprehensive income (loss)	70		86,710
Total other comprehensive income	1,350	1,786	6,210
Balance at end of period	2,567,667	2,009,188	2,142,733
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	(365)	(2,151)	(8,361)
Balance at end of period	985	(365)	(2,151)
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	0	(6,551)
Other comprehensive income (loss) before reclassifications	0	0	(80,175)
Amounts reclassified from accumulated other comprehensive income (loss)	0		86,726
Total other comprehensive income	0	0	6,551
Balance at end of period	0	0	0
Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	(365)	(2,151)	(1,810)
Other comprehensive income (loss) before reclassifications	1,280	1,786	(325)
Amounts reclassified from accumulated other comprehensive income (loss)	70		(16)
Total other comprehensive income	1,350	1,786	(341)
Balance at end of period	$ 985	$ (365)	$ (2,151)