CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 42,116	$ 160,574
Net income, Tax Effect	(3,595)	(40,388)
Net income	38,521	120,186
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	158,341	102,785
Reclassification adjustment for net losses realized in net income, Before-tax Amount	(8,734)	(75)
Net unrealized gains (losses), Before-tax Amount	149,607	102,710
Net unrealized gains (losses) on investment securities available for sale: Tax Effect
Net unrealized holding gains (losses) arising during the period, Tax Effect	(41,011)	(26,621)
Reclassification adjustment for net losses realized in net income, Tax Effect	2,262	19
Net unrealized gains (losses), Tax Effect	(38,749)	(26,602)
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	117,330	76,164
Reclassification adjustment for net losses realized in net income, Net of Tax Amount	(6,472)	(56)
Net unrealized gains (losses), Net of Tax Amount	110,858	76,108
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	108,639	0
Reclassification adjustment for realized (gains) losses included in net income, Before-tax Amount	(120)	0
Net change, Before-tax Amount	108,519	0
Unrealized gains (losses) on derivative instruments designated as cash flow hedges, Tax Effect
Net unrealized gains (losses) arising during the period, Income Tax	28,138	0
Reclassification adjustment for realized (gains) losses included in net income, Income Tax	31	0
Net change, Income Tax	(28,107)	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	80,501	0
Reclassification adjustment for realized (gains) losses included in net income, Net of Tax Amount	89	0
Net change, Net of Tax Amount	80,412	0
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for realized (gains) losses included in net income, Before-tax Amount	0	(35)
Post-retirement unfunded health benefit: Tax Effect
Reclassification adjustment for realized (gains) losses included in net income, Tax Effect	0	5
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for realized (gains) losses included in net income, Net of Tax Amount	0	(30)
Other comprehensive income (loss), Before-tax Amount	258,126	102,675
Other comprehensive income (loss), Tax Effect	(66,856)	(26,597)
Other comprehensive income (loss), Net of Tax Amount	191,270	76,078
Comprehensive income, Net of Tax Amount	$ 229,791	$ 196,264