CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 142,117	$ 117,791	$ 275,494	$ 223,496
Net income, Tax (Expense) Benefit	(30,936)	(41,788)	(61,146)	(75,635)
Net income	111,181	76,003	214,348	147,861
Net change related to cash flow hedges:
Reclassification adjustment for losses realized in net income, Before-tax Amount	0	65	0	130
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	0	(25)	0	(50)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	0	40	0	80
Net unrealized gains on investment securities available for sale:
Reclassification adjustment for net losses (gains) realized in net income, Before-tax Amount	1,296	1	1,296	(7,667)
Net unrealized gains arising during the period, Before-tax Amount	(25,476)	11,150	(86,921)	20,250
Net unrealized gain, Before-tax Amount	(24,180)	11,151	(85,625)	12,583
Net unrealized gains on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net losses (gains) realized in net income, Tax (Expense) Benefit	(336)	0	(336)	2,952
Net unrealized gains arising during the period, Tax (Expense) Benefit	6,598	(4,293)	22,512	(7,797)
Net unrealized gains, Tax (Expense) Benefit	6,262	(4,293)	22,176	(4,845)
Net unrealized gains on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net losses (gains) realized in net income, Net of Tax Amount	960	1	960	(4,715)
Net unrealized gains arising during the period, Net of Tax Amount	(18,878)	6,857	(64,409)	12,453
Net unrealized gains, Net of Tax Amount	(17,918)	6,858	(63,449)	7,738
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(34)	(20)	(68)	(40)
Net unrealized (realized) gains, Before-tax Amount	(34)	(20)	(68)	(40)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	9	8	22	16
Net unrealized (realized) gains, Tax (Expense) Benefit	9	8	22	16
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(25)	(12)	(46)	(24)
Net unrealized (realized) gain, Net of Tax Amount	(25)	(12)	(46)	(24)
Other comprehensive income (loss), Before-tax Amount	(24,214)	11,196	(85,693)	12,673
Other comprehensive income (loss), Tax (Expense) Benefit	6,271	(4,310)	22,198	(4,879)
Other comprehensive income (loss), Net of Tax Amount	(17,943)	6,886	(63,495)	7,794
Comprehensive income, Net of Tax Amount	$ 93,238	$ 82,889	$ 150,853	$ 155,655