Other Comprehensive Income (Loss) (Details) - 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
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment gains (losses) arising during the year									$ 69,052	$ (5,425)	$ (33,718)
Income tax (expense) benefit									(21,505)	1,776	11,738
Valuation allowance									0	0	62,383
Net of taxes									47,547	(3,649)	40,403
Net changes in benefit plan assets and obligations									(8,983)	(14,799)	(12,818)
Income tax benefit									3,144	5,179	4,487
Valuation allowance									0	0	(7,383)
Net of taxes									(5,839)	(9,620)	(15,714)
Net changes in unrealized foreign currency translation adjustment									45	(1,463)	(5,699)
Income tax (expense) benefit									(14)	512	2,000
Valuation allowance									0	0	(529)
Net of taxes									31	(951)	(4,228)
Total other comprehensive income (loss)									60,114	(21,687)	(52,235)
Total income tax (expense) benefit, net of valuation allowance									(18,375)	7,467	72,696
Other comprehensive income (loss), net of tax									41,739	(14,220)	20,461
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									784,496	514,714	487,687
Total income tax benefit (expense), net of valuation allowance	$ (218,142)	$ (64,440)	$ (61,994)	$ (84,159)	$ (54,551)	$ (27,131)	$ (56,018)	$ (34,497)	(428,735)	(172,197)	684,313
Valuation allowance											161,158
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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				2,548,842				2,236,140	2,548,842	2,236,140
Balance, end of year	3,154,526				2,548,842				3,154,526	2,548,842	2,236,140
Accumulated other comprehensive loss
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive income (loss), net of tax									41,739	(14,220)	20,461
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(75,100)				(60,880)	(75,100)	(60,880)	(81,341)
Other comprehensive income (loss) before reclassifications									40,651	(8,147)	33,707
Less: Amounts reclassified from AOCL									(1,088)	6,073	13,246
Less: Amounts reclassified for lower enacted corporate tax rate	10,400								10,422
Balance, end of year	(43,783)				(75,100)				(43,783)	(75,100)	(60,880)
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(20,797)				(17,148)	(20,797)	(17,148)	(57,551)
Other comprehensive income (loss) before reclassifications									45,870	508	51,655
Less: Amounts reclassified from AOCL									(1,677)	4,157	11,252
Less: Amounts reclassified for lower enacted corporate tax rate									(2,525)
Balance, end of year	29,275				(20,797)				29,275	(20,797)	(17,148)
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(54,272)				(44,652)	(54,272)	(44,652)	(28,938)
Other comprehensive income (loss) before reclassifications									(5,250)	(8,658)	(13,720)
Less: Amounts reclassified from AOCL									589	962	1,994
Less: Amounts reclassified for lower enacted corporate tax rate									12,947
Balance, end of year	(73,058)				(54,272)				(73,058)	(54,272)	(44,652)
Net unrealized foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				$ (31)				$ 920	(31)	920	5,148
Other comprehensive income (loss) before reclassifications									31	3	(4,228)
Less: Amounts reclassified from AOCL									0	954	0
Less: Amounts reclassified for lower enacted corporate tax rate									0
Balance, end of year	$ 0				$ (31)				0	(31)	920
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									(1,674)	9,154	13,877
Total income tax benefit (expense), net of valuation allowance									586	(3,081)	(631)
Net income									(1,088)	6,073	13,246
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									(2,580)	6,207	11,693
Total income tax benefit (expense), net of valuation allowance									903	(2,050)	(4,076)
Valuation allowance									0	0	3,635
Net income									(1,677)	4,157	11,252
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									906	1,480	2,184
Total income tax benefit (expense), net of valuation allowance									(317)	(518)	(764)
Valuation allowance									0	0	574
Net income									589	962	1,994
Reclassification from Accumulated Other Comprehensive Income | Net unrealized foreign currency translation
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									0	1,467	0
Total income tax benefit (expense), net of valuation allowance									0	(513)	0
Net income									$ 0	$ 954	$ 0