Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 358,573	$ 302,559	$ 252,628
Net income, Tax (Expense) Benefit	132,491	107,310	93,245
Net income	226,082	195,249	159,383
Net change related to cash flow hedges: Before-tax Amount
Reclassification adjustment for losses (gains) realized in net income, Before-tax Amount	521	448	447
Net change related to cash flow hedges: Tax (Expense) Benefit
Reclassification adjustment for losses (gains) realized in net income, Tax (Expense) Benefit	(201)	(173)	(173)
Net change related to cash flow hedges: Net of Tax Amount
Reclassification adjustment for losses (gains) realized in net income, Net of Tax Amount	320	275	274
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	(2,769)	(1,331)	(2,945)
Net unrealized gains (losses) arising during the period, Before-tax Amount	(25,707)	47,223	(71,929)
Net unrealized gains (losses), Before-tax Amount	(28,476)	45,892	(74,874)
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	1,066	513	1,134
Net unrealized gains (losses) arising during the period, Tax (Expense) Benefit	9,901	(18,182)	27,693
Net unrealized gains (losses), Tax (Expense) Benefit	10,967	(17,669)	28,827
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	(1,703)	(818)	(1,811)
Net unrealized gains (losses) arising during the period, Net of Tax Amount	(15,806)	29,041	(44,236)
Net unrealized gains (losses), Net of Tax Amount	(17,509)	28,223	(46,047)
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(272)	(144)	(170)
Actuarial gains arising during the period, Before-tax Amount	236	395	830
Net unrealized gains, Before-tax Amount	(36)	251	660
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	104	56	65
Actuarial gains arising during the period, Tax (Expense) Benefit	(93)	(152)	(311)
Net unrealized gains, Tax (Expense) Benefit	11	(96)	(246)
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(168)	(88)	(105)
Actuarial gains arising during the period, Net of Tax Amount	143	243	519
Net unrealized gains, Net of Tax Amount	(25)	155	414
Other comprehensive income (loss), Before-tax Amount	(27,991)	46,591	(73,767)
Other comprehensive income (loss), Tax (Expense) Benefit	10,777	(17,938)	28,408
Other comprehensive income (loss), Net of Tax Amount	(17,214)	28,653	(45,359)
Comprehensive income, Net of Tax Amount	$ 208,868	$ 223,902	$ 114,024