Securities (Continuous Unrealized Losses) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Fair Value
Less Than 12 Months	$ 1,065,508,000		$ 1,065,508,000		$ 942,845,000
12 Months or More	1,422,572,000		1,422,572,000		1,483,868,000
Total	2,488,080,000		2,488,080,000		2,426,713,000
Gross Unrealized Losses
Less Than 12 Months	(26,582,000)		(26,582,000)		(9,919,000)
12 Months or More	(67,632,000)		(67,632,000)		(48,409,000)
Total	(94,214,000)		(94,214,000)		$ (58,328,000)
Impairment loss	$ 0	$ 0	$ 0	$ 0
Number of securities in an unrealized loss position | security	193		193		165
U.S. Treasury securities
Fair Value
Less Than 12 Months	$ 192,840,000		$ 192,840,000		$ 168,061,000
12 Months or More	436,430,000		436,430,000		472,219,000
Total	629,270,000		629,270,000		640,280,000
Gross Unrealized Losses
Less Than 12 Months	(4,227,000)		(4,227,000)		(1,005,000)
12 Months or More	(14,774,000)		(14,774,000)		(10,110,000)
Total	$ (19,001,000)		$ (19,001,000)		$ (11,115,000)
Number of securities in an unrealized loss position | security	22		22		25
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	$ 141,270,000		$ 141,270,000		$ 99,935,000
12 Months or More	86,287,000		86,287,000		85,281,000
Total	227,557,000		227,557,000		185,216,000
Gross Unrealized Losses
Less Than 12 Months	(3,667,000)		(3,667,000)		(623,000)
12 Months or More	(1,028,000)		(1,028,000)		(2,862,000)
Total	$ (4,695,000)		$ (4,695,000)		$ (3,485,000)
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	113		113		98
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	$ 133,352,000		$ 133,352,000		$ 113,775,000
12 Months or More	189,400,000		189,400,000		191,827,000
Total	322,752,000		322,752,000		305,602,000
Gross Unrealized Losses
Less Than 12 Months	(3,911,000)		(3,911,000)		(2,071,000)
12 Months or More	(11,409,000)		(11,409,000)		(7,461,000)
Total	(15,320,000)		(15,320,000)		(9,532,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities
Fair Value
Less Than 12 Months	467,178,000		467,178,000		413,621,000
12 Months or More	317,707,000		317,707,000		361,809,000
Total	784,885,000		784,885,000		775,430,000
Gross Unrealized Losses
Less Than 12 Months	(10,405,000)		(10,405,000)		(4,205,000)
12 Months or More	(13,241,000)		(13,241,000)		(9,357,000)
Total	(23,646,000)		(23,646,000)		(13,562,000)
Municipal securities
Fair Value
Less Than 12 Months	24,453,000		24,453,000		8,490,000
12 Months or More	8,334,000		8,334,000		8,588,000
Total	32,787,000		32,787,000		17,078,000
Gross Unrealized Losses
Less Than 12 Months	(755,000)		(755,000)		(123,000)
12 Months or More	(428,000)		(428,000)		(186,000)
Total	(1,183,000)		(1,183,000)		(309,000)
Non-agency residential mortgage-backed securities | Investment grade
Fair Value
Less Than 12 Months	7,835,000		7,835,000		4,599,000
12 Months or More	0		0		0
Total	7,835,000		7,835,000		4,599,000
Gross Unrealized Losses
Less Than 12 Months	(169,000)		(169,000)		(22,000)
12 Months or More	0		0		0
Total	(169,000)		(169,000)		(22,000)
Corporate debt securities | Investment grade
Fair Value
Less Than 12 Months	9,754,000		9,754,000		0
12 Months or More	1,247,000		1,247,000		11,905,000
Total	11,001,000		11,001,000		11,905,000
Gross Unrealized Losses
Less Than 12 Months	(246,000)		(246,000)		0
12 Months or More	(3,000)		(3,000)		(746,000)
Total	(249,000)		(249,000)		(746,000)
Foreign bonds | Investment grade
Fair Value
Less Than 12 Months	76,266,000		76,266,000		103,149,000
12 Months or More	383,167,000		383,167,000		352,239,000
Total	459,433,000		459,433,000		455,388,000
Gross Unrealized Losses
Less Than 12 Months	(3,158,000)		(3,158,000)		(1,325,000)
12 Months or More	(26,749,000)		(26,749,000)		(17,687,000)
Total	$ (29,907,000)		$ (29,907,000)		$ (19,012,000)
Number of securities in an unrealized loss position | security	16		16		16
Asset-backed securities | Investment grade
Fair Value
Less Than 12 Months	$ 12,560,000		$ 12,560,000
12 Months or More	0		0
Total	12,560,000		12,560,000
Gross Unrealized Losses
Less Than 12 Months	(44,000)		(44,000)
12 Months or More	0		0
Total	$ (44,000)		$ (44,000)
Other securities
Fair Value
Less Than 12 Months					$ 31,215,000
12 Months or More					0
Total					31,215,000
Gross Unrealized Losses
Less Than 12 Months					(545,000)
12 Months or More					0
Total					$ (545,000)