Investment Securities Available for Sale and Investment Securities Held to Maturity - Adjusted cost and fair value of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	$ 11,228,605	$ 11,112,700
Available-for-sale Securities, Gross Unrealized Gain	84,243	29,432
Available-for-sale Securities, Gross Unrealized Losses	47,051	91,612
Investment securities available for sale	11,265,797	11,050,520
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	1,267,953	1,322,676
Held-to-maturity Securities, Gross Unrealized Gain	10,421	12,397
Held-to-maturity Securities, Gross Unrealized Losses	76,542	90,952
Investment securities held to maturity, estimated fair value	1,201,832	1,244,121
U.S. Treasury and other U.S. government agencies
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	2,702,503	3,232,238
Available-for-sale Securities, Gross Unrealized Gain	29,519	4,076
Available-for-sale Securities, Gross Unrealized Losses	12,676	24,822
Investment securities available for sale	2,719,346	3,211,492
Mortgage-backed securities
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	4,562,638	4,624,441
Available-for-sale Securities, Gross Unrealized Gain	31,203	16,548
Available-for-sale Securities, Gross Unrealized Losses	23,016	50,727
Investment securities available for sale	4,570,825	4,590,262
Collateralized mortgage obligations
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	3,474,587	2,713,075
Available-for-sale Securities, Gross Unrealized Gain	22,901	8,200
Available-for-sale Securities, Gross Unrealized Losses	11,327	16,019
Investment securities available for sale	3,486,161	2,705,256
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	98,642	103,947
Held-to-maturity Securities, Gross Unrealized Gain	4,435	6,022
Held-to-maturity Securities, Gross Unrealized Losses	8,111	4,634
Investment securities held to maturity, estimated fair value	94,966	105,335
State and political subdivisions
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	14,043	15,492
Available-for-sale Securities, Gross Unrealized Gain	242	395
Available-for-sale Securities, Gross Unrealized Losses	0	0
Investment securities available for sale	14,285	15,887
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	1,081,227	1,128,240
Held-to-maturity Securities, Gross Unrealized Gain	1,407	729
Held-to-maturity Securities, Gross Unrealized Losses	64,644	82,632
Investment securities held to maturity, estimated fair value	1,017,990	1,046,337
Other
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	19,054	23,914
Available-for-sale Securities, Gross Unrealized Gain	336	175
Available-for-sale Securities, Gross Unrealized Losses	32	44
Investment securities available for sale	19,358	24,045
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	66,637	66,478
Held-to-maturity Securities, Gross Unrealized Gain	2,115	2,644
Held-to-maturity Securities, Gross Unrealized Losses	2,130	2,112
Investment securities held to maturity, estimated fair value	66,622	67,010
Equity Securities
Investment securities available for sale:
Available-for-sale Securities, Amortized Cost Basis	455,780	503,540
Available-for-sale Securities, Gross Unrealized Gain	42	38
Available-for-sale Securities, Gross Unrealized Losses	0	0
Investment securities available for sale	455,822	503,578
Asset-backed securities
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	21,447	24,011
Held-to-maturity Securities, Gross Unrealized Gain	2,464	3,002
Held-to-maturity Securities, Gross Unrealized Losses	1,657	1,574
Investment securities held to maturity, estimated fair value	$ 22,254	$ 25,439

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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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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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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

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