Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment gains (losses) arising during the year										$ 220,139	$ (81,834)	$ 69,052
Income tax (expense) benefit										(46,229)	17,188	(21,505)
Net of taxes										173,910	(64,646)	47,547
Net changes in benefit plan assets and obligations										29,129	(19,958)	(8,983)
Income tax (expense) benefit										(6,117)	4,191	3,144
Net of taxes										23,012	(15,767)	(5,839)
Net changes in unrealized foreign currency translation adjustment										0	0	45
Income tax benefit (expense)										0	0	(14)
Net of taxes										0	0	31
Total other comprehensive income (loss)										249,268	(101,792)	60,114
Total income tax (expense) benefit, net										(52,346)	21,379	(18,375)
Other comprehensive income (loss), net of tax										196,922	(80,413)	41,739
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications										847,977	844,150	784,496
Income tax (expense) benefit	$ (45,599)	$ (46,186)	$ (43,433)	$ (38,996)	$ (36,963)	$ (49,994)	$ (50,708)	$ (36,388)		(174,214)	(174,053)	(428,735)
Net income	177,110	$ 176,934	$ 167,778	151,941	157,746	$ 181,900	$ 186,814	143,637		673,763	670,097	355,761
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				3,581,891				3,154,526		3,581,891	3,154,526
Less: Amounts reclassified for lower enacted corporate tax rate												10,422
Balance, end of year	4,309,234				3,581,891				$ 3,154,526	4,309,234	3,581,891	3,154,526
Accumulated other comprehensive loss
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive income (loss), net of tax										196,922	(80,413)	41,739
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(124,214)				(43,783)		(124,214)	(43,783)	(75,100)
Cumulative effect of adopting the accounting standard update for financial instruments									(18)			(18)
Other comprehensive income (loss) before reclassifications										193,400	(87,736)	40,651
Less: Amounts reclassified from AOCL										(3,522)	(7,323)	(1,088)
Less: Amounts reclassified for lower enacted corporate tax rate									10,400			10,422
Balance, end of year	72,708				(124,214)				(43,783)	72,708	(124,214)	(43,783)
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(35,389)				29,275		(35,389)	29,275	(20,797)
Cumulative effect of adopting the accounting standard update for financial instruments									(18)			(18)
Other comprehensive income (loss) before reclassifications										176,784	(70,206)	45,870
Less: Amounts reclassified from AOCL										2,874	(5,560)	(1,677)
Less: Amounts reclassified for lower enacted corporate tax rate												(2,525)
Balance, end of year	138,521				(35,389)				29,275	138,521	(35,389)	29,275
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				(88,825)				(73,058)		(88,825)	(73,058)	(54,272)
Other comprehensive income (loss) before reclassifications										16,616	(17,530)	(5,250)
Less: Amounts reclassified from AOCL										(6,396)	(1,763)	589
Less: Amounts reclassified for lower enacted corporate tax rate												12,947
Balance, end of year	(65,813)				(88,825)				(73,058)	(65,813)	(88,825)	(73,058)
Net unrealized foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				$ 0				$ 0		0	0	(31)
Other comprehensive income (loss) before reclassifications										0	0	31
Less: Amounts reclassified from AOCL										0	0	0
Less: Amounts reclassified for lower enacted corporate tax rate												0
Balance, end of year	$ 0				$ 0				$ 0	0	0	0
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications										(4,460)	(9,269)	(1,674)
Income tax (expense) benefit										938	1,946	586
Net income										(3,522)	(7,323)	(1,088)
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										3,637	(7,037)	(2,580)
Income tax (expense) benefit										(763)	1,477	903
Net income										2,874	(5,560)	(1,677)
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										(8,097)	(2,232)	906
Income tax (expense) benefit										1,701	469	(317)
Net income										$ (6,396)	$ (1,763)	$ 589