Investment Securities Available for Sale and Investment Securities Held to Maturity - Fair value and unrealized losses of available for sale securities and held to maturity securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	$ 1,667,887	$ 6,026,410
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	12,758	47,281
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	2,635,622	2,262,517
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	34,293	44,331
Available-for-sale securities, fair value	4,303,509	8,288,927
Available-for-sale securities, unrealized losses	47,051	91,612
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	33,550	84,368
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	3,779	6,859
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	896,578	866,895
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	72,763	84,093
Held-to-maturity securities, fair value	930,128	951,263
Held-to-maturity securities unrealized losses	76,542	90,952
U.S. Treasury and other U.S. government agencies
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	267,701	2,081,528
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	2,479	16,523
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	566,514	460,160
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	10,197	8,299
Available-for-sale securities, fair value	834,215	2,541,688
Available-for-sale securities, unrealized losses	12,676	24,822
Mortgage-backed securities
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	597,351	2,623,761
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	3,571	20,380
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,640,437	1,408,069
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	19,445	30,347
Available-for-sale securities, fair value	2,237,788	4,031,830
Available-for-sale securities, unrealized losses	23,016	50,727
Collateralized mortgage obligations
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	802,835	1,321,121
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	6,708	10,378
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	427,581	393,210
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	4,619	5,641
Available-for-sale securities, fair value	1,230,416	1,714,331
Available-for-sale securities, unrealized losses	11,327	16,019
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	13,981	11,066
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	1,698	326
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	48,902	52,601
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	6,413	4,308
Held-to-maturity securities, fair value	62,883	63,667
Held-to-maturity securities unrealized losses	8,111	4,634
State and political subdivisions
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	19,569	73,302
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	2,081	6,533
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	829,864	794,489
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	62,563	76,099
Held-to-maturity securities, fair value	849,433	867,791
Held-to-maturity securities unrealized losses	64,644	82,632
Other
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	0	0
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	0	0
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,090	1,078
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	32	44
Available-for-sale securities, fair value	1,090	1,078
Available-for-sale securities, unrealized losses	32	44
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	0	0
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	0	0
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	3,983	4,015
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	2,130	2,112
Held-to-maturity securities, fair value	3,983	4,015
Held-to-maturity securities unrealized losses	2,130	2,112
Asset-backed securities
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	0	0
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	0	0
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	13,829	15,790
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	1,657	1,574
Held-to-maturity securities, fair value	13,829	15,790
Held-to-maturity securities unrealized losses	$ 1,657	$ 1,574