Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	$ 91,523	$ (12,067)	$ 196,233	$ (93,029)
Income tax (expense) benefit	(19,220)	2,533	(41,209)	19,536
Other comprehensive income (loss), net of tax	72,303	(9,534)	155,024	(73,493)
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	211,211	237,522	402,147	417,547
Income tax (expense) benefit	(43,433)	(50,708)	(82,428)	(87,096)
Net income	167,778	186,814	319,719	330,451
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			3,581,891
Other comprehensive income before reclassifications			151,053
Less: Amounts reclassified from AOCL			(3,971)
Balance, end of period	4,035,665	3,313,875	4,035,665	3,313,875
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(260)	(4,112)	(5,028)	(4,828)
Income tax (expense) benefit	55	863	1,057	1,013
Net income	(205)	(3,249)	(3,971)	(3,815)
Accumulated other comprehensive income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income (loss), net of tax	72,303	(9,534)	155,024	(73,493)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(41,493)	(107,760)	(124,214)	(43,801)
Balance, end of period	30,810	(117,294)	30,810	(117,294)
Net unrealized gains and (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	89,562	(12,558)	192,183	(94,145)
Income tax (expense) benefit	(18,808)	2,636	(40,358)	19,770
Other comprehensive income (loss), net of tax	70,754	(9,922)	151,825	(74,375)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(35,389)
Other comprehensive income before reclassifications			151,053
Less: Amounts reclassified from AOCL			(772)
Balance, end of period	116,436		116,436
Net unrealized gains and (losses) on available-for-sale securities | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	1,701	(3,621)	(978)	(3,712)
Income tax (expense) benefit	(357)	760	206	779
Net income	1,344	(2,861)	(772)	(2,933)
Net benefit plan assets and (obligations) recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	1,961	491	4,050	1,116
Income tax (expense) benefit	(412)	(103)	(851)	(234)
Other comprehensive income (loss), net of tax	1,549	388	3,199	882
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(88,825)
Other comprehensive income before reclassifications			0
Less: Amounts reclassified from AOCL			(3,199)
Balance, end of period	(85,626)		(85,626)
Net benefit plan assets and (obligations) recognized in shareholders' equity | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(1,961)	(491)	(4,050)	(1,116)
Income tax (expense) benefit	412	103	851	234
Net income	$ (1,549)	$ (388)	$ (3,199)	$ (882)