Securities (Details 2)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Fair Value
Less Than 12 Months	$ 1,128,863,000		$ 1,128,863,000		$ 2,162,896,000
12 Months or More	976,729,000		976,729,000		253,038,000
Total	2,105,592,000		2,105,592,000		2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(10,521,000)		(10,521,000)		(51,760,000)
12 Months or More	(31,816,000)		(31,816,000)		(4,581,000)
Total	(42,337,000)		(42,337,000)		$ (56,341,000)
Impairment loss	$ 0	$ 0	$ 0	$ 0
Number of securities in an unrealized loss position | security	146		146		170
Gross Unrealized Losses
Carrying amount of investment security held-to-maturity transferred to available-for-sale	$ 115,600,000		$ 115,600,000
Fair Value
Less Than 12 Months	1,128,863,000		1,128,863,000		$ 2,162,896,000
12 Months or More	976,729,000		976,729,000		253,038,000
Total	2,105,592,000		2,105,592,000		2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(10,521,000)		(10,521,000)		(51,760,000)
12 Months or More	(31,816,000)		(31,816,000)		(4,581,000)
Total	(42,337,000)		(42,337,000)		(56,341,000)
U.S. Treasury securities
Fair Value
Less Than 12 Months	415,507,000		415,507,000		670,268,000
12 Months or More	110,825,000		110,825,000		0
Total	526,332,000		526,332,000		670,268,000
Gross Unrealized Losses
Less Than 12 Months	(4,615,000)		(4,615,000)		(9,829,000)
12 Months or More	(2,088,000)		(2,088,000)		0
Total	$ (6,703,000)		$ (6,703,000)		$ (9,829,000)
Number of securities in an unrealized loss position | security	22		22		26
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	$ 96,681,000		$ 96,681,000		$ 203,901,000
12 Months or More	54,512,000		54,512,000		0
Total	151,193,000		151,193,000		203,901,000
Gross Unrealized Losses
Less Than 12 Months	(367,000)		(367,000)		(3,249,000)
12 Months or More	(2,256,000)		(2,256,000)		0
Total	$ (2,623,000)		$ (2,623,000)		$ (3,249,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities
Gross Unrealized Losses
Number of securities in an unrealized loss position | security	79		79		82
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	$ 120,070,000		$ 120,070,000		$ 202,106,000
12 Months or More	155,128,000		155,128,000		29,201,000
Total	275,198,000		275,198,000		231,307,000
Gross Unrealized Losses
Less Than 12 Months	(1,721,000)		(1,721,000)		(5,452,000)
12 Months or More	(5,376,000)		(5,376,000)		(766,000)
Total	(7,097,000)		(7,097,000)		(6,218,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities
Fair Value
Less Than 12 Months	365,038,000		365,038,000		629,324,000
12 Months or More	288,768,000		288,768,000		119,603,000
Total	653,806,000		653,806,000		748,927,000
Gross Unrealized Losses
Less Than 12 Months	(2,344,000)		(2,344,000)		(9,594,000)
12 Months or More	(5,538,000)		(5,538,000)		(1,955,000)
Total	(7,882,000)		(7,882,000)		(11,549,000)
Municipal securities
Fair Value
Less Than 12 Months	22,010,000		22,010,000		57,655,000
12 Months or More	11,256,000		11,256,000		2,692,000
Total	33,266,000		33,266,000		60,347,000
Gross Unrealized Losses
Less Than 12 Months	(222,000)		(222,000)		(1,699,000)
12 Months or More	(234,000)		(234,000)		(201,000)
Total	(456,000)		(456,000)		(1,900,000)
Non-agency residential mortgage-backed securities | Investment grade
Fair Value
Less Than 12 Months	4,715,000		4,715,000		5,033,000
12 Months or More	0		0		6,444,000
Total	4,715,000		4,715,000		11,477,000
Gross Unrealized Losses
Less Than 12 Months	(7,000)		(7,000)		(101,000)
12 Months or More	0		0		(14,000)
Total	(7,000)		(7,000)		(115,000)
Corporate debt securities | Investment grade
Fair Value
Less Than 12 Months	0		0		0
12 Months or More	2,327,000		2,327,000		71,667,000
Total	2,327,000		2,327,000		71,667,000
Gross Unrealized Losses
Less Than 12 Months	0		0		0
12 Months or More	(137,000)		(137,000)		(375,000)
Total	(137,000)		(137,000)		(375,000)
Corporate debt securities | Non-investment grade
Fair Value
Less Than 12 Months	0		0		0
12 Months or More	9,615,000		9,615,000		9,173,000
Total	9,615,000		9,615,000		9,173,000
Gross Unrealized Losses
Less Than 12 Months	0		0		0
12 Months or More	(576,000)		(576,000)		(1,018,000)
Total	(576,000)		(576,000)		(1,018,000)
Foreign bonds | Investment grade
Fair Value
Less Than 12 Months	73,619,000		73,619,000		363,618,000
12 Months or More	344,298,000		344,298,000		14,258,000
Total	417,917,000		417,917,000		377,876,000
Gross Unrealized Losses
Less Than 12 Months	(873,000)		(873,000)		(21,327,000)
12 Months or More	(15,611,000)		(15,611,000)		(252,000)
Total	$ (16,484,000)		$ (16,484,000)		$ (21,579,000)
Number of securities in an unrealized loss position | security	14		14		13
Other securities
Fair Value
Less Than 12 Months	$ 31,223,000		$ 31,223,000		$ 30,991,000
12 Months or More	0		0		0
Total	31,223,000		31,223,000		30,991,000
Gross Unrealized Losses
Less Than 12 Months	(372,000)		(372,000)		(509,000)
12 Months or More	0		0		0
Total	(372,000)		(372,000)		(509,000)
Non-agency commercial mortgage-backed security
Fair Value
Less Than 12 Months	0		0		0
12 Months or More	0		0		0
Total	0		0		0
Gross Unrealized Losses
Less Than 12 Months	0		0		0
12 Months or More	0		0		0
Total	$ 0		$ 0		$ 0