Investment Securities (Amort cost, gross unrealized gains/losses, and fair value) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-Sale:
Amortized Cost	$ 2,236,092	$ 2,262,807
Gross Unrealized Gains	45,534	41,668
Gross Unrealized Losses	(10,440)	(15,285)
Available-for-Sale	2,271,186us-gaap_AvailableForSaleSecurities	2,289,190us-gaap_AvailableForSaleSecurities
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	4,306,353	4,466,679
Gross Unrealized Gains	83,479	66,859
Gross Unrealized Losses	(11,825)	(29,043)
Held-to-Maturity Securities, Fair Value	4,378,007	4,504,495
Debt Securities Issued by the U.S. Treasury and Government Agencies
Available-for-Sale:
Amortized Cost	350,177	325,365
Gross Unrealized Gains	5,610	5,933
Gross Unrealized Losses	(136)	(40)
Available-for-Sale	355,651	331,258
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	518,849	498,767
Gross Unrealized Gains	4,592	2,008
Gross Unrealized Losses	(69)	(1,159)
Held-to-Maturity Securities, Fair Value	523,372	499,616
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Amortized Cost	727,980	723,474
Gross Unrealized Gains	25,068	21,941
Gross Unrealized Losses	(580)	(1,445)
Available-for-Sale	752,468	743,970
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	248,674	249,559
Gross Unrealized Gains	16,435	15,459
Gross Unrealized Losses	0	0
Held-to-Maturity Securities, Fair Value	265,109	265,018
Debt Securities Issued by Corporations
Available-for-Sale:
Amortized Cost	288,225	298,272
Gross Unrealized Gains	665	546
Gross Unrealized Losses	(2,945)	(3,985)
Available-for-Sale	285,945	294,833
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	162,867	166,686
Gross Unrealized Gains	1,948	109
Gross Unrealized Losses	(913)	(3,442)
Held-to-Maturity Securities, Fair Value	163,902	163,353
Mortgage-Backed Securities
Available-for-Sale:
Amortized Cost	869,710	915,696
Gross Unrealized Gains	14,191	13,248
Gross Unrealized Losses	(6,779)	(9,815)
Available-for-Sale	877,122	919,129
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	3,375,963	3,551,667
Gross Unrealized Gains	60,504	49,283
Gross Unrealized Losses	(10,843)	(24,442)
Held-to-Maturity Securities, Fair Value	3,425,624	3,576,508
Residential - Government Agencies
Available-for-Sale:
Amortized Cost	417,962	452,493
Gross Unrealized Gains	10,325	10,986
Gross Unrealized Losses	(896)	(1,043)
Available-for-Sale	427,391	462,436
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	2,706,237	2,862,369
Gross Unrealized Gains	50,307	45,407
Gross Unrealized Losses	(10,118)	(20,636)
Held-to-Maturity Securities, Fair Value	2,746,426	2,887,140
Residential - U.S. Government Sponsored Enterprises
Available-for-Sale:
Amortized Cost	279,708	276,390
Gross Unrealized Gains	3,866	2,262
Gross Unrealized Losses	0	(191)
Available-for-Sale	283,574	278,461
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	361,273	379,365
Gross Unrealized Gains	6,145	3,635
Gross Unrealized Losses	0	(15)
Held-to-Maturity Securities, Fair Value	367,418	382,985
Commercial - Government Agencies
Available-for-Sale:
Amortized Cost	172,040	186,813
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5,883)	(8,581)
Available-for-Sale	166,157	178,232
Held-to-Maturity:
Held-to-maturity Securities, Amortized Cost	308,453	309,933
Gross Unrealized Gains	4,052	241
Gross Unrealized Losses	(725)	(3,791)
Held-to-Maturity Securities, Fair Value	$ 311,780	$ 306,383

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Fair value of investments in debt securities classified as held-to-maturity.

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