Debt Securities Available for Sale and Debt 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, 2018	Dec. 31, 2017
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	$ 4,372,820	$ 4,166,961
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	85,137	37,561
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	5,183,808	6,289,298
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	179,304	174,112
Available-for-sale securities, fair value	9,556,628	10,456,259
Available-for-sale securities, unrealized losses	264,441	211,673
Debt securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	68,656	265,031
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	862	4,488
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	1,267,646	390,508
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	25,007	12,265
Held-to-maturity securities, fair value	1,336,302	655,539
Held-to-maturity securities unrealized losses	25,869	16,753
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	2,842,522	2,532,439
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	68,207	28,308
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,347,030	1,325,975
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	48,346	33,546
Available-for-sale securities, fair value	4,189,552	3,858,414
Available-for-sale securities, unrealized losses	116,553	61,854
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	439,002	390,106
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	5,363	2,731
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,575,272	1,666,045
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	48,769	40,365
Available-for-sale securities, fair value	2,014,274	2,056,151
Available-for-sale securities, unrealized losses	54,132	43,096
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	1,091,296	1,244,416
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	11,567	6,522
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	2,261,506	3,297,278
Available-for-sale securities, Securities in a loss position for 12 months or longer, unrealized losses	82,189	100,201
Available-for-sale securities, fair value	3,352,802	4,541,694
Available-for-sale securities, unrealized losses	93,756	106,723
Agency
Debt securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	0
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	0
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	934,348
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	14,829
Held-to-maturity securities, fair value	934,348
Held-to-maturity securities unrealized losses	14,829
Non-agency
Debt securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	2,003	9,776
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	257	25
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	23,039	22,439
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	2,334	1,580
Held-to-maturity securities, fair value	25,042	32,215
Held-to-maturity securities unrealized losses	2,591	1,605
State and political subdivisions
Debt securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	66,653	236,207
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	605	4,365
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	291,117	341,090
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	6,769	8,488
Held-to-maturity securities, fair value	357,770	577,297
Held-to-maturity securities unrealized losses	7,374	12,853
Other
Debt securities held to maturity:
Held-to-maturity securities, securities in a loss position for less than 12 months, fair value	0	19,048
Held-to-maturity securities, securities in a loss position for less than 12 months, unrealized losses	0	98
Held-to-maturity securities, securities in a loss position for 12 months or longer, fair value	13,389	20,736
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	535	1,569
Held-to-maturity securities, fair value	13,389	39,784
Held-to-maturity securities unrealized losses	535	1,667
Asset-backed securities
Debt 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	5,753	6,243
Held-to-maturity securities, securities in a loss position for 12 months or longer, unrealized losses	540	628
Held-to-maturity securities, fair value	5,753	6,243
Held-to-maturity securities unrealized losses	$ 540	$ 628