Investment Securities Available for Sale and Investment Securities Held to Maturity - Adjusted cost and fair value of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	$ 11,972,790	$ 11,833,978
Available-for-sale Securities, Gross Unrealized Gain	25,919	20,825
Available-for-sale Securities, Gross Unrealized Losses	152,091	189,748
Investment securities available for sale	11,846,618	11,665,055
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	1,158,785	1,203,217
Held-to-maturity Securities, Gross Unrealized Gain	11,070	10,699
Held-to-maturity Securities, Gross Unrealized Losses	27,161	31,907
Investment securities held to maturity, estimated fair value	1,142,694	1,182,009
U.S. Treasury and other U.S. government agencies
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	2,995,084	2,409,141
Available-for-sale Securities, Gross Unrealized Gain	4,896	2,390
Available-for-sale Securities, Gross Unrealized Losses	32,595	37,200
Investment securities available for sale	2,967,385	2,374,331
Mortgage-backed securities
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	3,541,581	3,796,270
Available-for-sale Securities, Gross Unrealized Gain	14,965	12,869
Available-for-sale Securities, Gross Unrealized Losses	34,522	45,801
Investment securities available for sale	3,522,024	3,763,338
Collateralized mortgage obligations
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	5,013,180	5,200,241
Available-for-sale Securities, Gross Unrealized Gain	5,796	5,292
Available-for-sale Securities, Gross Unrealized Losses	84,833	106,605
Investment securities available for sale	4,934,143	5,098,928
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	77,791	83,087
Held-to-maturity Securities, Gross Unrealized Gain	4,926	5,265
Held-to-maturity Securities, Gross Unrealized Losses	3,424	3,278
Investment securities held to maturity, estimated fair value	79,293	85,074
State and political subdivisions
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	8,089	8,457
Available-for-sale Securities, Gross Unrealized Gain	144	184
Available-for-sale Securities, Gross Unrealized Losses	25	0
Investment securities available for sale	8,208	8,641
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	1,002,928	1,040,716
Held-to-maturity Securities, Gross Unrealized Gain	3,302	2,309
Held-to-maturity Securities, Gross Unrealized Losses	20,680	25,518
Investment securities held to maturity, estimated fair value	985,550	1,017,507
Other
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	14,129	16,321
Available-for-sale Securities, Gross Unrealized Gain	48	6
Available-for-sale Securities, Gross Unrealized Losses	116	142
Investment securities available for sale	14,061	16,185
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	64,752	64,296
Held-to-maturity Securities, Gross Unrealized Gain	928	1,143
Held-to-maturity Securities, Gross Unrealized Losses	2,101	2,030
Investment securities held to maturity, estimated fair value	63,579	63,409
Equity Securities
Investment securities available for sale:
Available-for-sale securities, amortized cost basis	400,727	403,548
Available-for-sale Securities, Gross Unrealized Gain	70	84
Available-for-sale Securities, Gross Unrealized Losses	0	0
Investment securities available for sale	400,797	403,632
Asset-backed securities
Investment securities held to maturity:
Held-to-maturity Securities, Amortized Cost Basis	13,314	15,118
Held-to-maturity Securities, Gross Unrealized Gain	1,914	1,982
Held-to-maturity Securities, Gross Unrealized Losses	956	1,081
Investment securities held to maturity, estimated fair value	$ 14,272	$ 16,019

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
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-URI http://asc.fasb.org/extlink&oid=65884222&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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Fair value of investments in debt securities classified as held-to-maturity.

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