Debt Securities Available for Sale and Debt Securities Held to Maturity - Adjusted cost and fair value of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt securities available for sale:
Available-for-sale securities, amortized cost basis	$ 11,681,055	$ 12,411,689
Available-for-sale Securities, Gross Unrealized Gain	17,538	19,616
Available-for-sale Securities, Gross Unrealized Losses	264,441	211,673
Debt securities available for sale	11,434,152	12,219,632
Debt securities held to maturity:
Total	1,975,729	1,046,093
Held-to-maturity Securities, Gross Unrealized Gain	18,136	11,203
Held-to-maturity Securities, Gross Unrealized Losses	25,869	16,753
Debt securities held to maturity, estimated fair value	1,967,996	1,040,543
Available-for-sale securities transferred to Held-to-maturity securities	1,000,000
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, amortized cost basis	4,908,132	4,265,296
Available-for-sale Securities, Gross Unrealized Gain	2,753	996
Available-for-sale Securities, Gross Unrealized Losses	116,553	61,854
Debt securities available for sale	4,794,332	4,204,438
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, amortized cost basis	2,666,049	2,841,584
Available-for-sale Securities, Gross Unrealized Gain	9,961	14,312
Available-for-sale Securities, Gross Unrealized Losses	54,132	43,096
Debt securities available for sale	2,621,878	2,812,800
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, amortized cost basis	4,105,306	5,302,531
Available-for-sale Securities, Gross Unrealized Gain	4,739	4,203
Available-for-sale Securities, Gross Unrealized Losses	93,756	106,723
Debt securities available for sale	4,016,289	5,200,011
Agency
Debt securities held to maturity:
Total	949,177
Held-to-maturity Securities, Gross Unrealized Gain	0
Held-to-maturity Securities, Gross Unrealized Losses	14,829
Debt securities held to maturity, estimated fair value	934,348
Non-agency
Debt securities held to maturity:
Total	60,491	64,140
Held-to-maturity Securities, Gross Unrealized Gain	5,896	5,262
Held-to-maturity Securities, Gross Unrealized Losses	2,591	1,605
Debt securities held to maturity, estimated fair value	63,796	67,797
State and political subdivisions
Debt securities available for sale:
Available-for-sale securities, amortized cost basis	1,568	2,278
Available-for-sale Securities, Gross Unrealized Gain	85	105
Available-for-sale Securities, Gross Unrealized Losses	0	0
Debt securities available for sale	1,653	2,383
Debt securities held to maturity:
Total	897,084	911,393
Held-to-maturity Securities, Gross Unrealized Gain	9,183	3,951
Held-to-maturity Securities, Gross Unrealized Losses	7,374	12,853
Debt securities held to maturity, estimated fair value	898,893	902,491
Other
Debt securities held to maturity:
Total	60,661	61,252
Held-to-maturity Securities, Gross Unrealized Gain	1,371	243
Held-to-maturity Securities, Gross Unrealized Losses	535	1,667
Debt securities held to maturity, estimated fair value	61,497	59,828
Asset-backed securities
Debt securities held to maturity:
Total	8,316	9,308
Held-to-maturity Securities, Gross Unrealized Gain	1,686	1,747
Held-to-maturity Securities, Gross Unrealized Losses	540	628
Debt securities held to maturity, estimated fair value	$ 9,462	$ 10,427

Available-for-sale Securities, Transfers to Held-to-maturity, Carrying Value

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

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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