CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 131,363	$ 186,985	$ 297,536	$ 558,383
Net income, Tax Effect	(39,789)	(51,259)	(74,250)	(146,287)
Net income	91,574	135,726	223,286	412,096
Net unrealized gains (losses) on investment securities available for sale: Before-tax Amount
Net unrealized holding gains (losses) arising during the period, Before-tax Amount	(29,428)	33,919	116,975	226,160
Reclassification adjustment for net losses realized in net income, Before-tax Amount	1,550	3,731	(76,594)	5,502
Net unrealized gains (losses), Before-tax Amount	(27,878)	37,650	40,381	231,662
Net unrealized gains (losses) on investment securities available for sale: Tax Effect
Net unrealized holding gains (losses) arising during the period, Tax Effect	7,622	(8,786)	(30,297)	(58,574)
Reclassification adjustment for net losses realized in net income, Tax Effect	(401)	(966)	19,838	(1,425)
Net unrealized gains (losses), Tax Effect	7,221	(9,752)	(10,459)	(59,999)
Net unrealized gains (losses) on investment securities available for sale: Net of Tax Amount
Net unrealized holding gains (losses) arising during the period, Net of Tax Amount	(21,806)	25,133	86,678	167,586
Reclassification adjustment for net losses realized in net income, Net of Tax Amount	1,149	2,765	(56,756)	4,077
Net unrealized gains (losses), Net of Tax Amount	(20,657)	27,898	29,922	171,663
Unrealized gains (losses) on derivative instruments designated as cash flow hedges, Before-tax Amount
Net unrealized gains (losses) arising during the period, Before-tax Amount	(8,954)	(1,182)	108,508	(1,182)
Reclassification adjustment for realized (gains) losses included in net income, Before-tax Amount	(1,031)	0	(1,421)	0
Net change, Before-tax Amount	(9,985)	(1,182)	107,087	(1,182)
Unrealized gains (losses) on derivative instruments designated as cash flow hedges, Tax Effect
Net unrealized gains (losses) arising during the period, Income Tax	2,319	306	(28,104)	306
Reclassification adjustment for realized (gains) losses included in net income, Income Tax	267	0	368	0
Net change, Income Tax	2,586	306	(27,736)	306
Unrealized gains (losses) on derivative instruments designated as cash flow hedges, Net of Tax Amount
Net unrealized gains (losses) arising during the period, Net of Tax Amount	(6,635)	(876)	80,404	(876)
Reclassification adjustment for realized (gains) losses included in net income, Net of Tax Amount	(764)	0	(1,053)	0
Net change, Net of Tax Amount	(7,399)	(876)	79,351	(876)
Post-retirement unfunded health benefit: Before-tax Amount
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	0	(510)	0	(510)
Reclassification adjustment for realized (gains) losses included in net income, Before-tax Amount	0	0	0	(70)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax	0	(510)	0	(580)
Post-retirement unfunded health benefit: Tax Effect
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax	0	(132)	0	(132)
Reclassification adjustment for realized (gains) losses included in net income, Tax Effect	0	0	0	14
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	0	132	0	146
Post-retirement unfunded health benefit: Net of Tax Amount
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax	0	(378)	0	(378)
Reclassification adjustment for realized (gains) losses included in net income, Net of Tax Amount	0	0	0	(56)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	0	(378)	0	(434)
Other comprehensive income (loss), Before-tax Amount	(37,863)	35,958	147,468	229,900
Other comprehensive income (loss), Tax Effect	9,807	(9,314)	(38,195)	(59,547)
Other comprehensive income (loss), net of income taxes	(28,056)	26,644	109,273	170,353
Comprehensive income, Net of Tax Amount	$ 63,518	$ 162,370	$ 332,559	$ 582,449