Accumulated Other Comprehensive Income - Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ 2,088,159	$ 2,523,905	$ 2,567,667
Net current-period other comprehensive income	(3,077)	2,360	1,092
Balance at end of period	2,151,487	2,088,159	2,523,905
Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	4,437	2,077	985
Other comprehensive income before reclassifications	(2,530)	2,461	2,815
Amounts reclassified from accumulated other comprehensive loss	547	101	1,723
Net current-period other comprehensive income	(3,077)	2,360	1,092
Balance at end of period	$ 1,360	$ 4,437	$ 2,077