Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 388,451	$ 358,573	$ 302,559
Net income, Tax (Expense) Benefit	(141,667)	(132,491)	(107,310)
Net income	246,784	226,082	195,249
Net change related to cash flow hedges: Before-tax Amount
Reclassification adjustment for losses realized in net income, Before-tax Amount	467	521	448
Net change related to cash flow hedges: Tax (Expense) Benefit
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(180)	(201)	(173)
Net change related to cash flow hedges: Net of Tax Amount
Reclassification adjustment for losses realized in net income, Net of Tax Amount	287	320	275
Net unrealized gains (losses) on investment securities available for sale: Before-tax Amount
Reclassification adjustment for net gains realized in net income, Before-tax Amount	(6,011)	(2,769)	(1,331)
Net unrealized (losses) gains arising during the period, Before-tax Amount	(36,432)	(25,707)	47,223
Net unrealized gains (losses), Before-tax Amount	(42,443)	(28,476)	45,892
Net unrealized gains (losses) on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net gains realized in net income, Tax (Expense) Benefit	2,314	1,066	513
Net unrealized (losses) gains arising during the period, Tax (Expense) Benefit	14,027	9,901	(18,182)
Net unrealized gains (losses), Tax (Expense) Benefit	16,341	10,967	(17,669)
Net unrealized gains (losses) on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net gains realized in net income, Net of Tax Amount	(3,697)	(1,703)	(818)
Net unrealized (losses) gains arising during the period, Net of Tax Amount	(22,405)	(15,806)	29,041
Net unrealized gains (losses), Net of Tax Amount	(26,102)	(17,509)	28,223
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(144)	(272)	(144)
Actuarial gains arising during the period, Before-tax Amount	102	236	395
Net unrealized (losses) gains, Before-tax Amount	(42)	(36)	251
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	56	104	56
Actuarial gains arising during the period, Tax (Expense) Benefit	(39)	(93)	(152)
Net unrealized (losses) gains, Tax (Expense) Benefit	17	11	(96)
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(88)	(168)	(88)
Actuarial gains arising during the period, Net of Tax Amount	63	143	243
Net unrealized (losses) gains, Net of Tax Amount	(25)	(25)	155
Other comprehensive (loss) income, Before-tax Amount	(42,018)	(27,991)	46,591
Other comprehensive (loss) income, Tax (Expense) Benefit	16,178	10,777	(17,938)
Other comprehensive (loss) income, Net of Tax Amount	(25,840)	(17,214)	28,653
Comprehensive income, Net of Tax Amount	$ 220,944	$ 208,868	$ 223,902