CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 355,761	$ 342,517	$ 1,172,000
Other comprehensive income (loss), net of tax:
Change in unrealized investment gains and losses									47,547	(3,649)	40,403
Benefit plans adjustment									(5,839)	(9,620)	(15,714)
Foreign currency translation adjustment									31	(951)	(4,228)
Other comprehensive income (loss), net of tax									41,739	(14,220)	20,461
Comprehensive income									$ 397,500	$ 328,297	$ 1,192,461