Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 105,704	$ 83,730
Net income, Tax (Expense) Benefit	(33,847)	(31,199)
Net income	71,857	52,531
Reclassification adjustment for losses realized in net income, Before-tax Amount	65	273
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(25)	(105)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	40	168
Net unrealized gains on investment securities available for sale:
Reclassification adjustment for net gains realized in net income, Before-tax Amount	(7,668)	(67)
Net unrealized gains arising during the period, Before-tax Amount	9,099	47,172
Net unrealized gain, Before-tax Amount	1,431	47,105
Net unrealized gains on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net gains realized in net income, Tax (Expense) Benefit	2,952	26
Net unrealized gains arising during the period, Tax (Expense) Benefit	(3,503)	(18,162)
Net unrealized gains, Tax (Expense) Benefit	(551)	(18,136)
Net unrealized gains on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net gains realized in net income, Net of Tax Amount	(4,716)	(41)
Net unrealized gains arising during the period, Net of Tax Amount	5,596	29,010
Net unrealized gains, Net of Tax Amount	880	28,969
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(20)	(94)
Actuarial gains arising during the period, Before-tax Amount	0	0
Net unrealized (realized) gains, Before-tax Amount	(20)	(94)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	8	36
Actuarial gains arising during the period, Tax (Expense) Benefit	0	0
Net unrealized (realized) gains, Tax (Expense) Benefit	8	36
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(12)	(58)
Actuarial gains arising during the period, Net of Tax Amount	0	0
Net unrealized (realized) gains, Net of Tax Amount	(12)	(58)
Other comprehensive income, Before-tax Amount	1,476	47,284
Other comprehensive income, Tax (Expense) Benefit	(568)	(18,205)
Other comprehensive income, Net of Tax Amount	908	29,079
Comprehensive income, Net of Tax Amount	$ 72,765	$ 81,610