Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment (losses) gains arising during the year									$ (81,834)	$ 69,052	$ (5,425)
Income tax benefit (expense)									17,188	(21,505)	1,776
Net of taxes									(64,646)	47,547	(3,649)
Net changes in benefit plan assets and obligations									(19,958)	(8,983)	(14,799)
Income tax benefit									4,191	3,144	5,179
Net of taxes									(15,767)	(5,839)	(9,620)
Net changes in unrealized foreign currency translation adjustment									0	45	(1,463)
Income tax (expense) benefit									0	(14)	512
Net of taxes									0	31	(951)
Total other comprehensive (loss) income									(101,792)	60,114	(21,687)
Total income tax benefit (expense), net									21,379	(18,375)	7,467
Other comprehensive (loss) income, net of tax									(80,413)	41,739	(14,220)
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									844,150	784,496	514,714
Total income tax benefit (expense), net	$ (36,963)	$ (49,994)	$ (50,708)	$ (36,388)	$ (218,142)	$ (64,440)	$ (61,994)	$ (84,159)	(174,053)	(428,735)	(172,197)
Net income	157,746	$ 181,900	$ 186,814	143,637	27,314	$ 120,027	$ 118,622	89,798	670,097	355,761	342,517
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				3,154,526				2,548,842	3,154,526	2,548,842
Balance, end of year	3,581,891				3,154,526				3,581,891	3,154,526	2,548,842
Accumulated other comprehensive loss
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive (loss) income, net of tax									(80,413)	41,739	(14,220)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(43,783)				(75,100)	(43,783)	(75,100)	(60,880)
Cumulative effect of accounting standard update					(18)					(18)
Other comprehensive income (loss) before reclassifications									(87,736)	40,651	(8,147)
Less: Amounts reclassified from AOCL									(7,323)	(1,088)	6,073
Less: Amounts reclassified for lower enacted corporate tax rate					10,400					10,422
Balance, end of year	(124,214)				(43,783)				(124,214)	(43,783)	(75,100)
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				29,275				(20,797)	29,275	(20,797)	(17,148)
Cumulative effect of accounting standard update					(18)					(18)
Other comprehensive income (loss) before reclassifications									(70,206)	45,870	508
Less: Amounts reclassified from AOCL									(5,560)	(1,677)	4,157
Less: Amounts reclassified for lower enacted corporate tax rate										(2,525)
Balance, end of year	(35,389)				29,275				(35,389)	29,275	(20,797)
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(73,058)				(54,272)	(73,058)	(54,272)	(44,652)
Other comprehensive income (loss) before reclassifications									(17,530)	(5,250)	(8,658)
Less: Amounts reclassified from AOCL									(1,763)	589	962
Less: Amounts reclassified for lower enacted corporate tax rate										12,947
Balance, end of year	(88,825)				(73,058)				(88,825)	(73,058)	(54,272)
Net unrealized foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				$ 0				$ (31)	0	(31)	920
Other comprehensive income (loss) before reclassifications									0	31	3
Less: Amounts reclassified from AOCL									0	0	954
Less: Amounts reclassified for lower enacted corporate tax rate										0
Balance, end of year	$ 0				$ 0				0	0	(31)
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									(9,269)	(1,674)	9,154
Total income tax benefit (expense), net									1,946	586	(3,081)
Net income									(7,323)	(1,088)	6,073
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									(7,037)	(2,580)	6,207
Total income tax benefit (expense), net									1,477	903	(2,050)
Net income									(5,560)	(1,677)	4,157
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									(2,232)	906	1,480
Total income tax benefit (expense), net									469	(317)	(518)
Net income									(1,763)	589	962
Reclassification from Accumulated Other Comprehensive Income | Net unrealized foreign currency translation
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									0	0	1,467
Total income tax benefit (expense), net									0	0	(513)
Net income									$ 0	$ 0	$ 954