CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 133,375	$ 105,704
Net income, Tax (Expense) Benefit	(30,209)	(33,847)
Net income	103,166	71,857
Reclassification adjustment for losses realized in net income, Before-tax Amount	0	65
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	0	(25)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	0	40
Net unrealized gains on investment securities available for sale:
Reclassification adjustment for net losses (gains) realized in net income, Before-tax Amount	0	(7,668)
Net unrealized gains arising during the period, Before-tax Amount	(61,445)	9,099
Net unrealized gain, Before-tax Amount	(61,445)	1,431
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	0	2,952
Net unrealized gains arising during the period, Tax (Expense) Benefit	15,914	(3,503)
Net unrealized gains, Tax (Expense) Benefit	15,914	(551)
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	0	(4,716)
Net unrealized gains arising during the period, Net of Tax Amount	(45,531)	5,596
Net unrealized gains, Net of Tax Amount	(45,531)	880
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(34)	(20)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	13	8
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(21)	(12)
Other comprehensive income (loss), Before-tax Amount	(61,479)	1,476
Other comprehensive income (loss), Tax (Expense) Benefit	15,927	(568)
Other comprehensive income (loss), Net of Tax Amount	(45,552)	908
Comprehensive income, Net of Tax Amount	$ 57,614	$ 72,765