CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 102,418	$ 822,852	$ 113,654	$ 133,076	$ 342,517	$ 1,172,000	$ 251,949
Other comprehensive (loss) income, net of tax:
Change in unrealized investment gains and losses									(3,649)	40,403	91,139
Benefit plans adjustment									(9,620)	(15,714)	(52,112)
Foreign currency translation adjustment									(951)	(4,228)	(2,642)
Other comprehensive (loss) income, net of tax									(14,220)	20,461	36,385
Comprehensive income									$ 328,297	$ 1,192,461	$ 288,334