Accumulated Other Comprehensive Income - Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ 2,151,487	$ 2,088,159	$ 2,523,905
Total other comprehensive income	1,059	(3,077)	2,360
Balance at end of period	2,215,707	2,151,487	2,088,159
Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	1,360	4,437	2,077
Other comprehensive income before reclassifications	1,634	(2,530)	2,461
Amounts reclassified from accumulated other comprehensive loss	(575)	(547)	(101)
Total other comprehensive income	1,059	(3,077)	2,360
Balance at end of period	$ 2,419	$ 1,360	$ 4,437