Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	$ 41,736	$ (14,730)	$ 237,969	$ (107,759)
Income tax (expense) benefit	(8,764)	3,093	(49,973)	22,629
Other comprehensive income (loss), net of tax	32,972	(11,637)	187,996	(85,130)
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	223,120	231,894	625,267	649,441
Income tax (expense) benefit	(46,186)	(49,994)	(128,614)	(137,090)
Net income	176,934	181,900	496,653	512,351
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			3,581,891
Other comprehensive income before reclassifications			184,615
Less: Amounts reclassified from AOCI			(3,381)
Balance, end of period	4,159,617	3,489,531	4,159,617	3,489,531
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	748	(3,125)	(4,280)	(7,953)
Income tax (expense) benefit	(158)	657	899	1,670
Net income	590	(2,468)	(3,381)	(6,283)
Accumulated other comprehensive income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income (loss), net of tax	32,972	(11,637)	187,996	(85,130)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	30,810	(117,294)	(124,214)	(43,801)
Balance, end of period	63,782	(128,931)	63,782	(128,931)
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)	39,712	(15,288)	231,895	(109,433)
Income tax (expense) benefit	(8,340)	3,211	(48,698)	22,981
Other comprehensive income (loss), net of tax	31,372	(12,077)	183,197	(86,452)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(35,389)
Other comprehensive income before reclassifications			184,615
Less: Amounts reclassified from AOCI			1,418
Balance, end of period	147,808		147,808
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	2,772	(2,567)	1,794	(6,279)
Income tax (expense) benefit	(582)	539	(376)	1,318
Net income	2,190	(2,028)	1,418	(4,961)
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)	2,024	558	6,074	1,674
Income tax (expense) benefit	(424)	(118)	(1,275)	(352)
Other comprehensive income (loss), net of tax	1,600	440	4,799	1,322
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 AOCI			(4,799)
Balance, end of period	(84,026)		(84,026)
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	(2,024)	(558)	(6,074)	(1,674)
Income tax (expense) benefit	424	118	1,275	352
Net income	$ (1,600)	$ (440)	$ (4,799)	$ (1,322)