Securities (Continuous Unrealized Losses) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Fair Value
Less Than 12 Months	$ 1,045,676,000		$ 942,845,000
12 Months or More	1,444,107,000		1,483,868,000
Total	2,489,783,000		2,426,713,000
Gross Unrealized Losses
Less Than 12 Months	(22,728,000)		(9,919,000)
12 Months or More	(59,460,000)		(48,409,000)
Total	(82,188,000)		$ (58,328,000)
Impairment loss	$ 0	$ 0
Number of securities in an unrealized loss position | security	192		165
U.S. Treasury securities
Fair Value
Less Than 12 Months	$ 170,103,000		$ 168,061,000
12 Months or More	457,952,000		472,219,000
Total	628,055,000		640,280,000
Gross Unrealized Losses
Less Than 12 Months	(3,130,000)		(1,005,000)
12 Months or More	(13,803,000)		(10,110,000)
Total	$ (16,933,000)		$ (11,115,000)
Number of securities in an unrealized loss position | security	23		25
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	$ 133,861,000		$ 99,935,000
12 Months or More	86,549,000		85,281,000
Total	220,410,000		185,216,000
Gross Unrealized Losses
Less Than 12 Months	(2,157,000)		(623,000)
12 Months or More	(778,000)		(2,862,000)
Total	$ (2,935,000)		$ (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	111		98
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	$ 117,057,000		$ 113,775,000
12 Months or More	192,874,000		191,827,000
Total	309,931,000		305,602,000
Gross Unrealized Losses
Less Than 12 Months	(3,255,000)		(2,071,000)
12 Months or More	(10,061,000)		(7,461,000)
Total	(13,316,000)		(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	461,197,000		413,621,000
12 Months or More	339,157,000		361,809,000
Total	800,354,000		775,430,000
Gross Unrealized Losses
Less Than 12 Months	(8,362,000)		(4,205,000)
12 Months or More	(12,264,000)		(9,357,000)
Total	(20,626,000)		(13,562,000)
Municipal securities
Fair Value
Less Than 12 Months	24,618,000		8,490,000
12 Months or More	8,323,000		8,588,000
Total	32,941,000		17,078,000
Gross Unrealized Losses
Less Than 12 Months	(671,000)		(123,000)
12 Months or More	(446,000)		(186,000)
Total	(1,117,000)		(309,000)
Non-agency residential mortgage-backed securities | Investment grade
Fair Value
Less Than 12 Months	8,404,000		4,599,000
12 Months or More	0		0
Total	8,404,000		4,599,000
Gross Unrealized Losses
Less Than 12 Months	(143,000)		(22,000)
12 Months or More	0		0
Total	(143,000)		(22,000)
Corporate debt securities | Investment grade
Fair Value
Less Than 12 Months	0		0
12 Months or More	10,742,000		11,905,000
Total	10,742,000		11,905,000
Gross Unrealized Losses
Less Than 12 Months	0		0
12 Months or More	(699,000)		(746,000)
Total	(699,000)		(746,000)
Foreign bonds | Investment grade
Fair Value
Less Than 12 Months	130,436,000		103,149,000
12 Months or More	348,510,000		352,239,000
Total	478,946,000		455,388,000
Gross Unrealized Losses
Less Than 12 Months	(5,010,000)		(1,325,000)
12 Months or More	(21,409,000)		(17,687,000)
Total	$ (26,419,000)		$ (19,012,000)
Number of securities in an unrealized loss position | security	17		16
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)