Securities (Details 2)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Fair Value
Less Than 12 Months	$ 1,661,833,000		$ 1,661,833,000		$ 2,162,896,000
12 Months or More	301,499,000		301,499,000		253,038,000
Total	1,963,332,000		1,963,332,000		2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(31,297,000)		(31,297,000)		(51,760,000)
12 Months or More	(10,133,000)		(10,133,000)		(4,581,000)
Total	(41,430,000)		(41,430,000)		$ (56,341,000)
Impairment loss	$ 0	$ 0	$ 0	$ 0
Number of securities in an unrealized loss position | security	140		140		170
Fair Value
Less Than 12 Months	$ 1,661,833,000		$ 1,661,833,000		$ 2,162,896,000
12 Months or More	301,499,000		301,499,000		253,038,000
Total	1,963,332,000		1,963,332,000		2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(31,297,000)		(31,297,000)		(51,760,000)
12 Months or More	(10,133,000)		(10,133,000)		(4,581,000)
Total	(41,430,000)		(41,430,000)		(56,341,000)
U.S. Treasury securities
Fair Value
Less Than 12 Months	536,842,000		536,842,000		670,268,000
12 Months or More	10,014,000		10,014,000		0
Total	546,856,000		546,856,000		670,268,000
Gross Unrealized Losses
Less Than 12 Months	(6,932,000)		(6,932,000)		(9,829,000)
12 Months or More	(6,000)		(6,000)		0
Total	$ (6,938,000)		$ (6,938,000)		$ (9,829,000)
Number of securities in an unrealized loss position | security	24		24		26
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	$ 134,583,000		$ 134,583,000		$ 203,901,000
12 Months or More	0		0		0
Total	134,583,000		134,583,000		203,901,000
Gross Unrealized Losses
Less Than 12 Months	(2,655,000)		(2,655,000)		(3,249,000)
12 Months or More	0		0		0
Total	$ (2,655,000)		$ (2,655,000)		$ (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	74		74		82
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	$ 213,675,000		$ 213,675,000		$ 202,106,000
12 Months or More	48,309,000		48,309,000		29,201,000
Total	261,984,000		261,984,000		231,307,000
Gross Unrealized Losses
Less Than 12 Months	(5,521,000)		(5,521,000)		(5,452,000)
12 Months or More	(1,215,000)		(1,215,000)		(766,000)
Total	(6,736,000)		(6,736,000)		(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	423,711,000		423,711,000		629,324,000
12 Months or More	124,052,000		124,052,000		119,603,000
Total	547,763,000		547,763,000		748,927,000
Gross Unrealized Losses
Less Than 12 Months	(4,904,000)		(4,904,000)		(9,594,000)
12 Months or More	(2,553,000)		(2,553,000)		(1,955,000)
Total	(7,457,000)		(7,457,000)		(11,549,000)
Municipal securities
Fair Value
Less Than 12 Months	17,652,000		17,652,000		57,655,000
12 Months or More	2,795,000		2,795,000		2,692,000
Total	20,447,000		20,447,000		60,347,000
Gross Unrealized Losses
Less Than 12 Months	(312,000)		(312,000)		(1,699,000)
12 Months or More	(92,000)		(92,000)		(201,000)
Total	$ (404,000)		$ (404,000)		(1,900,000)
Number of securities in an unrealized loss position | security	13		13
Non-agency residential mortgage-backed securities | Investment grade
Fair Value
Less Than 12 Months	$ 4,742,000		$ 4,742,000		5,033,000
12 Months or More	0		0		6,444,000
Total	4,742,000		4,742,000		11,477,000
Gross Unrealized Losses
Less Than 12 Months	(66,000)		(66,000)		(101,000)
12 Months or More	0		0		(14,000)
Total	(66,000)		(66,000)		(115,000)
Corporate debt securities | Investment grade
Fair Value
Less Than 12 Months	0		0		0
12 Months or More	2,296,000		2,296,000		71,667,000
Total	2,296,000		2,296,000		71,667,000
Gross Unrealized Losses
Less Than 12 Months	0		0		0
12 Months or More	(176,000)		(176,000)		(375,000)
Total	(176,000)		(176,000)		(375,000)
Corporate debt securities | Non-investment grade
Fair Value
Less Than 12 Months	0		0		0
12 Months or More	9,492,000		9,492,000		9,173,000
Total	9,492,000		9,492,000		9,173,000
Gross Unrealized Losses
Less Than 12 Months	0		0		0
12 Months or More	(699,000)		(699,000)		(1,018,000)
Total	(699,000)		(699,000)		(1,018,000)
Foreign bonds | Investment grade
Fair Value
Less Than 12 Months	299,434,000		299,434,000		363,618,000
12 Months or More	104,541,000		104,541,000		14,258,000
Total	403,975,000		403,975,000		377,876,000
Gross Unrealized Losses
Less Than 12 Months	(10,570,000)		(10,570,000)		(21,327,000)
12 Months or More	(5,392,000)		(5,392,000)		(252,000)
Total	$ (15,962,000)		$ (15,962,000)		$ (21,579,000)
Number of securities in an unrealized loss position | security	13		13		13
Other securities
Fair Value
Less Than 12 Months	$ 31,194,000		$ 31,194,000		$ 30,991,000
12 Months or More	0		0		0
Total	31,194,000		31,194,000		30,991,000
Gross Unrealized Losses
Less Than 12 Months	(337,000)		(337,000)		(509,000)
12 Months or More	0		0		0
Total	(337,000)		(337,000)		(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