Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ 2,567,667	$ 2,009,188	$ 2,142,733
Total other comprehensive income	1,092	1,350	1,786
Balance at end of period	2,523,905	2,567,667	2,009,188
Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	985	(365)	(2,151)
Other comprehensive income (loss) before reclassifications	2,815	1,280	1,786
Amounts reclassified from accumulated other comprehensive income (loss)	(1,723)	70
Total other comprehensive income	1,092	1,350	1,786
Balance at end of period	$ 2,077	$ 985	$ (365)