Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ 2,523,905	$ 2,567,667	$ 2,009,188
Total other comprehensive income	2,360	1,092	1,350
Balance at end of period	2,088,159	2,523,905	2,567,667
Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	2,077	985	(365)
Other comprehensive income (loss) before reclassifications	2,461	2,815	1,280
Amounts reclassified from accumulated other comprehensive income (loss)	(101)	(1,723)	70
Total other comprehensive income	2,360	1,092	1,350
Balance at end of period	$ 4,437	$ 2,077	$ 985