CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 151,430	$ 130,811	$ 14,047	$ 149,805	$ 177,110	$ 176,934	$ 167,778	$ 151,941	$ 446,093	$ 673,763	$ 670,097
Other comprehensive income (loss), net of tax:
Change in unrealized investment gains and losses									133,616	173,910	(64,646)
Benefit plans adjustment									10,497	23,012	(15,767)
Other comprehensive income (loss), net of tax									144,113	196,922	(80,413)
Comprehensive income									$ 590,206	$ 870,685	$ 589,684