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, 2017	Dec. 31, 2016
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	$ 5,533,529	$ 6,234,690
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	91,481	138,651
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	3,117,045	2,761,941
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	60,610	51,097
Available-for-sale securities, fair value	8,650,574	8,996,631
Available-for-sale securities, unrealized losses	152,091	189,748
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	440,061	552,858
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	10,192	13,745
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	371,267	366,711
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	16,969	18,162
Held-to-maturity securities, fair value	811,328	919,569
Held-to-maturity securities unrealized losses	27,161	31,907
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	1,342,663	1,277,341
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	13,129	23,862
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	747,898	609,078
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	19,466	13,338
Available-for-sale securities, fair value	2,090,561	1,886,419
Available-for-sale securities, unrealized losses	32,595	37,200
Mortgage-backed securities
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	1,202,675	1,425,743
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	9,804	15,235
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,293,140	1,368,957
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	24,718	30,566
Available-for-sale securities, fair value	2,495,815	2,794,700
Available-for-sale securities, unrealized losses	34,522	45,801
Collateralized mortgage obligations
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	2,979,987	3,527,757
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	68,468	99,477
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,074,947	782,849
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	16,365	7,128
Available-for-sale securities, fair value	4,054,934	4,310,606
Available-for-sale securities, unrealized losses	84,833	106,605
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	8,468	3,847
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	199	527
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	40,759	40,083
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	3,225	2,751
Held-to-maturity securities, fair value	49,227	43,930
Held-to-maturity securities unrealized losses	3,424	3,278
State and political subdivisions
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	4,333
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	25
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	0
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	0
Available-for-sale securities, fair value	4,333
Available-for-sale securities, unrealized losses	25
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	422,213	532,090
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	9,863	13,043
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	311,319	313,803
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	10,817	12,475
Held-to-maturity securities, fair value	733,532	845,893
Held-to-maturity securities unrealized losses	20,680	25,518
Other
Investment securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	3,871	3,849
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	55	77
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,060	1,057
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	61	65
Available-for-sale securities, fair value	4,931	4,906
Available-for-sale securities, unrealized losses	116	142
Investment securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	9,380	16,578
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	130	174
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	10,872	3,587
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	1,971	1,856
Held-to-maturity securities, fair value	20,252	20,165
Held-to-maturity securities unrealized losses	2,101	2,030
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	343
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	0	1
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	8,317	9,238
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	956	1,080
Held-to-maturity securities, fair value	8,317	9,581
Held-to-maturity securities unrealized losses	$ 956	$ 1,081