Securities (Continuous Unrealized Losses) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)
Fair Value
Less Than 12 Months	$ 942,845,000	$ 2,162,896,000
12 Months or More	1,483,868,000	253,038,000
Total	2,426,713,000	2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(9,919,000)	(51,760,000)
12 Months or More	(48,409,000)	(4,581,000)
Total	(58,328,000)	(56,341,000)
Impairment loss on available-for-sale securities	$ 0	$ 0	$ 0
Number of securities in an unrealized loss position | security	165	170
Fair Value
Less Than 12 Months	$ 942,845,000	$ 2,162,896,000
12 Months or More	1,483,868,000	253,038,000
Total	2,426,713,000	2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(9,919,000)	(51,760,000)
12 Months or More	(48,409,000)	(4,581,000)
Total	(58,328,000)	(56,341,000)
U.S. Treasury securities
Fair Value
Less Than 12 Months	168,061,000	670,268,000
12 Months or More	472,219,000	0
Total	640,280,000	670,268,000
Gross Unrealized Losses
Less Than 12 Months	(1,005,000)	(9,829,000)
12 Months or More	(10,110,000)	0
Total	$ (11,115,000)	$ (9,829,000)
Number of securities in an unrealized loss position | security	25	26
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	$ 99,935,000	$ 203,901,000
12 Months or More	85,281,000	0
Total	185,216,000	203,901,000
Gross Unrealized Losses
Less Than 12 Months	(623,000)	(3,249,000)
12 Months or More	(2,862,000)	0
Total	$ (3,485,000)	$ (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	98	82
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	$ 113,775,000	$ 202,106,000
12 Months or More	191,827,000	29,201,000
Total	305,602,000	231,307,000
Gross Unrealized Losses
Less Than 12 Months	(2,071,000)	(5,452,000)
12 Months or More	(7,461,000)	(766,000)
Total	(9,532,000)	(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	413,621,000	629,324,000
12 Months or More	361,809,000	119,603,000
Total	775,430,000	748,927,000
Gross Unrealized Losses
Less Than 12 Months	(4,205,000)	(9,594,000)
12 Months or More	(9,357,000)	(1,955,000)
Total	(13,562,000)	(11,549,000)
Municipal securities
Fair Value
Less Than 12 Months	8,490,000	57,655,000
12 Months or More	8,588,000	2,692,000
Total	17,078,000	60,347,000
Gross Unrealized Losses
Less Than 12 Months	(123,000)	(1,699,000)
12 Months or More	(186,000)	(201,000)
Total	(309,000)	(1,900,000)
Non-agency residential mortgage-backed securities | Investment grade
Fair Value
Less Than 12 Months	4,599,000	5,033,000
12 Months or More	0	6,444,000
Total	4,599,000	11,477,000
Gross Unrealized Losses
Less Than 12 Months	(22,000)	(101,000)
12 Months or More	0	(14,000)
Total	(22,000)	(115,000)
Corporate debt securities | Investment grade
Fair Value
Less Than 12 Months	0	0
12 Months or More	11,905,000	71,667,000
Total	11,905,000	71,667,000
Gross Unrealized Losses
Less Than 12 Months	0	0
12 Months or More	(746,000)	(375,000)
Total	(746,000)	(375,000)
Corporate debt securities | Non-investment grade
Fair Value
Less Than 12 Months		0
12 Months or More		9,173,000
Total		9,173,000
Gross Unrealized Losses
Less Than 12 Months		0
12 Months or More		(1,018,000)
Total		(1,018,000)
Foreign bonds | Investment grade
Fair Value
Less Than 12 Months	103,149,000	363,618,000
12 Months or More	352,239,000	14,258,000
Total	455,388,000	377,876,000
Gross Unrealized Losses
Less Than 12 Months	(1,325,000)	(21,327,000)
12 Months or More	(17,687,000)	(252,000)
Total	$ (19,012,000)	$ (21,579,000)
Number of securities in an unrealized loss position | security	16	13
Other securities
Fair Value
Less Than 12 Months	$ 31,215,000	$ 30,991,000
12 Months or More	0	0
Total	31,215,000	30,991,000
Gross Unrealized Losses
Less Than 12 Months	(545,000)	(509,000)
12 Months or More	0	0
Total	(545,000)	(509,000)
Non-agency commercial mortgage-backed security
Fair Value
Less Than 12 Months	0	0
12 months or More	0	0
Total	0	0
Gross Unrealized Losses
Less than 12 months	0	0
12 Months or More	0	0
Total	$ 0	$ 0