Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 88,034	$ 73,950	$ 173,847	$ 150,975
Net income, Tax (Expense) Benefit	(32,242)	(27,078)	(64,091)	(55,686)
Net income	55,792	46,872	109,756	95,289
Reclassification adjustment for losses realized in net income, Before-tax Amount	112	112	224	224
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(44)	(44)	(87)	(87)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	(68)	(68)	(137)	(137)
Net unrealized (losses) gains on investment securities available for sale:
Reclassification adjustment for net gains realized in net income, Before-tax Amount	(1,985)	0	(2,710)	(1,331)
Net unrealized (losses) gains arising during the period, Before-tax Amount	(28,678)	26,797	(13,467)	45,640
Net unrealized (losses) gains, Before-tax Amount	(30,663)	26,797	(16,177)	44,309
Net unrealized (losses) gains on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net gains realized in net income, Tax (Expense) Benefit	764	0	1,043	513
Net unrealized (losses) gains arising during the period, Tax (Expense) Benefit	11,042	(10,317)	5,188	(17,572)
Net unrealized (losses) gains, Tax (Expense) Benefit	11,806	(10,317)	6,231	(17,059)
Net unrealized (losses) gains on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net gains realized in net income, Net of Tax Amount	(1,221)	0	(1,667)	(818)
Net unrealized (losses) gains arising during the period, Net of Tax Amount	(17,636)	16,480	(8,279)	28,068
Net unrealized (losses) gains, Net of Tax Amount	(18,857)	16,480	(9,946)	27,250
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(42)	(72)	(84)	(144)
Actuarial gains arising during the period, Before-tax Amount	236	395	236	395
Net unrealized gains, Before-tax Amount	(194)	(323)	(152)	(251)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	16	28	32	56
Actuarial gains arising during the period, Tax (Expense) Benefit	(93)	(152)	(93)	(152)
Net unrealized gains, Tax (Expense) Benefit	(77)	(124)	(61)	(96)
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(26)	(44)	(52)	(88)
Actuarial gains arising during the period, Net of Tax Amount	143	243	143	243
Net unrealized gains, Net of Tax Amount	(117)	(199)	(91)	(155)
Other comprehensive (loss) income, Before-tax Amount	(30,357)	27,232	(15,801)	44,784
Other comprehensive (loss) income, Tax (Expense) Benefit	11,685	(10,485)	6,083	(17,242)
Other comprehensive (loss) income, Net of Tax Amount	(18,672)	16,747	(9,718)	27,542
Comprehensive income, Net of Tax Amount	$ 37,120	$ 63,619	$ 100,038	$ 122,831