CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 124,058	$ 210,824	$ 166,173	$ 371,398
Net income, Tax Effect	(30,866)	(54,640)	(34,461)	(95,028)
Net income	93,192	156,184	131,712	276,370
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	(11,939)	89,459	146,403	192,241
Reclassification adjustment for net losses realized in net income, Before-tax Amount	(69,409)	1,845	(78,144)	1,771
Net unrealized gains (losses), Before-tax Amount	(81,348)	91,304	68,259	194,012
Net unrealized gains (losses) on investment securities available for sale: Tax Effect
Net unrealized holding gains (losses) arising during the period, Tax Effect	3,092	(23,169)	(37,919)	(49,788)
Reclassification adjustment for net losses realized in net income, Tax Effect	17,977	(478)	20,239	(459)
Net unrealized gains (losses), Tax Effect	21,069	(23,647)	(17,680)	(50,247)
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	(8,847)	66,290	108,484	142,453
Reclassification adjustment for net losses realized in net income, Net of Tax Amount	(51,432)	1,367	(57,905)	1,312
Net unrealized gains (losses), Net of Tax Amount	(60,279)	67,657	50,579	143,765
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	8,823	0	117,462	0
Reclassification adjustment for realized (gains) losses included in net income, Before-tax Amount	(270)	0	(390)	0
Net change, Before-tax Amount	8,553	0	117,072	0
Unrealized gains (losses) on derivative instruments designated as cash flow hedges, Tax Effect
Net unrealized gains (losses) arising during the period, Income Tax	(2,285)	0	(30,423)	0
Reclassification adjustment for realized (gains) losses included in net income, Income Tax	70	0	101	0
Net change, Income Tax	(2,215)	0	(30,322)	0
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	6,538	0	87,039	0
Reclassification adjustment for realized (gains) losses included in net income, Net of Tax Amount	(200)	0	(289)	0
Net change, Net of Tax Amount	6,338	0	86,750	0
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for realized (gains) losses included in net income, Before-tax Amount	0	(35)	0	(70)
Post-retirement unfunded health benefit: Tax Effect
Reclassification adjustment for realized (gains) losses included in net income, Tax Effect	0	9	0	14
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for realized (gains) losses included in net income, Net of Tax Amount	0	(26)	0	(56)
Other comprehensive income (loss), Before-tax Amount	(72,795)	91,269	185,331	193,942
Other comprehensive income (loss), Tax Effect	18,854	(23,638)	(48,002)	(50,233)
Other comprehensive income (loss), Net of Tax Amount	(53,941)	67,631	137,329	143,709
Comprehensive income, Net of Tax Amount	$ 39,251	$ 223,815	$ 269,041	$ 420,079