Securities (Continuous Unrealized Losses) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 840,192,000		$ 840,192,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(20,163,000)		(20,163,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	1,660,596,000		1,660,596,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(92,829,000)		(92,829,000)
Available-for-sale debt securities Fair Value, Total	2,500,788,000		2,500,788,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(112,992,000)		(112,992,000)
Available-for-sale debt and equity investment securities, Less than 12 Months, Fair Value					$ 942,845,000
Available-for-sale debt and equity investment securities, Gross Unrealized Loss, Less than 12 Months					(9,919,000)
Available-for-sale debt and equity investment securities, More than 12 Months, Fair Value					1,483,868,000
Available-for-sale debt and equity investment securities, Gross Unrealized Loss, More than 12 Months					(48,409,000)
Available-for-sale debt and equityinvestment securities, Fair Value, Total					2,426,713,000
Available-for-sale debt and equity investment securities, Gross Unrealized Losses					$ (58,328,000)
Impairment loss, Available-for-sale debt securities	$ 0		$ 0
Impairment loss, Available-for-sale investment securities		$ 0		$ 0
Number of available-for-sale debt securities in an unrealized loss position | security	215		215
Number of available-for-sale investment securities in an unrealized loss position | security					165
U.S. Treasury securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 143,663,000		$ 143,663,000		$ 168,061,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(3,890,000)		(3,890,000)		(1,005,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	412,691,000		412,691,000		472,219,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(17,651,000)		(17,651,000)		(10,110,000)
Available-for-sale debt securities Fair Value, Total	556,354,000		556,354,000		640,280,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	$ (21,541,000)		$ (21,541,000)		$ (11,115,000)
Number of available-for-sale debt securities in an unrealized loss position | security	21		21
Number of available-for-sale investment securities in an unrealized loss position | security					25
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 108,267,000		$ 108,267,000		$ 99,935,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(2,531,000)		(2,531,000)		(623,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	120,389,000		120,389,000		85,281,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(3,978,000)		(3,978,000)		(2,862,000)
Available-for-sale debt securities Fair Value, Total	228,656,000		228,656,000		185,216,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	$ (6,509,000)		$ (6,509,000)		$ (3,485,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities
Fair Value and Gross Unrealized Losses
Number of available-for-sale debt securities in an unrealized loss position | security	127		127
Number of available-for-sale investment securities in an unrealized loss position | security					98
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 105,717,000		$ 105,717,000		$ 113,775,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(1,690,000)		(1,690,000)		(2,071,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	259,353,000		259,353,000		191,827,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(15,288,000)		(15,288,000)		(7,461,000)
Available-for-sale debt securities Fair Value, Total	365,070,000		365,070,000		305,602,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(16,978,000)		(16,978,000)		(9,532,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	347,135,000		347,135,000		413,621,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(7,135,000)		(7,135,000)		(4,205,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	460,718,000		460,718,000		361,809,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(22,053,000)		(22,053,000)		(9,357,000)
Available-for-sale debt securities Fair Value, Total	807,853,000		807,853,000		775,430,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(29,188,000)		(29,188,000)		(13,562,000)
Municipal securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	23,881,000		23,881,000		8,490,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(729,000)		(729,000)		(123,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	16,389,000		16,389,000		8,588,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(769,000)		(769,000)		(186,000)
Available-for-sale debt securities Fair Value, Total	40,270,000		40,270,000		17,078,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(1,498,000)		(1,498,000)		(309,000)
Non-agency commercial mortgage-backed securities | Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	15,926,000		15,926,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(104,000)		(104,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	0		0
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	0		0
Available-for-sale debt securities Fair Value, Total	15,926,000		15,926,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(104,000)		(104,000)
Non-agency residential mortgage-backed securities | Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	6,366,000		6,366,000		4,599,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(12,000)		(12,000)		(22,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	3,996,000		3,996,000		0
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(232,000)		(232,000)		0
Available-for-sale debt securities Fair Value, Total	10,362,000		10,362,000		4,599,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(244,000)		(244,000)		(22,000)
Corporate debt securities | Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	10,942,000		10,942,000		0
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(308,000)		(308,000)		0
Available-for-sale debt securities, More than 12 Months, Fair Value	0		0		11,905,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	0		0		(746,000)
Available-for-sale debt securities Fair Value, Total	10,942,000		10,942,000		11,905,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	(308,000)		(308,000)		(746,000)
Foreign bonds | Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	65,783,000		65,783,000		103,149,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(3,666,000)		(3,666,000)		(1,325,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	387,060,000		387,060,000		352,239,000
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(32,858,000)		(32,858,000)		(17,687,000)
Available-for-sale debt securities Fair Value, Total	452,843,000		452,843,000		455,388,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	$ (36,524,000)		$ (36,524,000)		$ (19,012,000)
Number of available-for-sale debt securities in an unrealized loss position | security	16		16
Number of available-for-sale investment securities in an unrealized loss position | security					16
Asset-backed securities | Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 12,512,000		$ 12,512,000
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(98,000)		(98,000)
Available-for-sale debt securities, More than 12 Months, Fair Value	0		0
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	0		0
Available-for-sale debt securities Fair Value, Total	12,512,000		12,512,000
Available-for-sale debt securities, Gross Unrealized Loss, Total	$ (98,000)		$ (98,000)
Other securities
Fair Value and Gross Unrealized Losses
Other securities, Less than 12 Months, Fair Value					$ 31,215,000
Other securities, Gross Unrealized Loss, Less than 12 Months					(545,000)
Other securities, More than 12 Months, Fair Value					0
Other securities, Gross Unrealized Loss, More than 12 Months					0
Other securities, Fair Value, Total					31,215,000
Other securities, Gross Unrealized Loss, Total					$ (545,000)