Other Comprehensive (Loss) Income (Details) - 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
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment (losses) gains arising during the year									$ (5,425)	$ (33,718)	$ 91,782
Income tax benefit (expense)									1,776	11,738	(32,017)
Valuation allowance									0	62,383	31,374
Net of taxes									(3,649)	40,403	91,139
Net changes in benefit plan assets and obligations									(14,799)	(12,818)	(52,112)
Income tax benefit									5,179	4,487	18,239
Valuation allowance									0	(7,383)	(18,239)
Net of taxes									(9,620)	(15,714)	(52,112)
Net changes in unrealized foreign currency translation adjustment									(1,463)	(5,699)	(4,067)
Income tax benefit									512	2,000	1,425
Valuation allowance									0	(529)	0
Net of taxes									(951)	(4,228)	(2,642)
Total other comprehensive (loss) income									(21,687)	(52,235)	35,603
Total income tax benefit, net of valuation allowance									7,467	72,696	782
Other comprehensive (loss) income, net of tax									(14,220)	20,461	36,385
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									514,714	487,687	254,723
Total income tax expense, net of valuation allowance	$ (54,551)	$ (27,131)	$ (56,018)	$ (34,497)	$ (6,584)	$ 695,604	$ (1,322)	$ (3,385)	(172,197)	684,313	(2,774)
Valuation allowance									0	161,158	88,833
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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				2,236,140				1,036,903	2,236,140	1,036,903
Balance, end of year	2,548,842				2,236,140				2,548,842	2,236,140	1,036,903
Accumulated other comprehensive loss
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive (loss) income, net of tax									(14,220)	20,461	36,385
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(60,880)				(81,341)	(60,880)	(81,341)	(117,726)
Other comprehensive income (loss) before reclassifications									(8,147)	33,707	36,399
Less: Amounts reclassified from AOCL									6,073	13,246	14
Balance, end of year	(75,100)				(60,880)				(75,100)	(60,880)	(81,341)
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(17,148)				(57,551)	(17,148)	(57,551)	(148,690)
Other comprehensive income (loss) before reclassifications									508	51,655	84,223
Less: Amounts reclassified from AOCL									4,157	11,252	(6,916)
Balance, end of year	(20,797)				(17,148)				(20,797)	(17,148)	(57,551)
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(44,652)				(28,938)	(44,652)	(28,938)	23,174
Other comprehensive income (loss) before reclassifications									(8,658)	(13,720)	(45,182)
Less: Amounts reclassified from AOCL									962	1,994	6,930
Balance, end of year	(54,272)				(44,652)				(54,272)	(44,652)	(28,938)
Net unrealized foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				$ 920				$ 5,148	920	5,148	7,790
Other comprehensive income (loss) before reclassifications									3	(4,228)	(2,642)
Less: Amounts reclassified from AOCL									954	0	0
Balance, end of year	$ (31)				$ 920				(31)	920	5,148
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									9,154	13,877	114
Total income tax expense, net of valuation allowance									(3,081)	(631)	(100)
Net income									6,073	13,246	14
Reclassification from Accumulated Other Comprehensive Income | Net unrealized gains and losses on available-for-sale securities
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									6,207	11,693	(6,816)
Total income tax expense, net of valuation allowance									(2,050)	(4,076)	2,402
Valuation allowance									0	3,635	(2,502)
Net income									4,157	11,252	(6,916)
Reclassification from Accumulated Other Comprehensive Income | Net benefit plan assets and obligations recognized in shareholders' equity
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									1,480	2,184	6,930
Total income tax expense, net of valuation allowance									(518)	(764)	(2,425)
Valuation allowance									0	574	2,425
Net income									962	1,994	6,930
Reclassification from Accumulated Other Comprehensive Income | Net unrealized foreign currency translation
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									1,467	0	0
Total income tax expense, net of valuation allowance									(513)	0	0
Net income									$ 954	$ 0	$ 0