Securities (Continuous Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 163,128
Available-for-sale debt securities, More than 12 Months, Fair Value	2,087,534
Available-for-sale debt securities, Fair Value, Total	2,250,662
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(1,518)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(69,249)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(70,767)
Available-for sale debt and equity investment securities, Less than 12 Months, Fair Value		$ 942,845
Available-for sale debt and equity investment securities, Gross Unrealized Losses, Less than 12 Months		(9,919)
Available-for sale debt and equity investment securities, More than 12 Months, Fair Value		1,483,868
Available-for sale debt and equity investment securities, Gross Unrealized Losses, More than 12 Months		(48,409)
Available-for sale debt and equity investment securities, Fair Value, Total		2,426,713
Available-for sale debt and equity investment securities, Gross Unrealized Losses, Total		(58,328)
U.S. Treasury securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	0	168,061
Available-for-sale debt securities, More than 12 Months, Fair Value	516,520	472,219
Available-for-sale debt securities, Fair Value, Total	516,520	640,280
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	0	(1,005)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(12,899)	(10,110)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(12,899)	(11,115)
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	22,755	99,935
Available-for-sale debt securities, More than 12 Months, Fair Value	159,814	85,281
Available-for-sale debt securities, Fair Value, Total	182,569	185,216
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(238)	(623)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(2,456)	(2,862)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(2,694)	(3,485)
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	26,886	113,775
Available-for-sale debt securities, More than 12 Months, Fair Value	274,666	191,827
Available-for-sale debt securities, Fair Value, Total	301,552	305,602
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(245)	(2,071)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(12,449)	(7,461)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(12,694)	(9,532)
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	75,675	413,621
Available-for-sale debt securities, More than 12 Months, Fair Value	653,660	361,809
Available-for-sale debt securities, Fair Value, Total	729,335	775,430
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(491)	(4,205)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(14,061)	(9,357)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(14,552)	(13,562)
Municipal securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	9,458	8,490
Available-for-sale debt securities, More than 12 Months, Fair Value	30,295	8,588
Available-for-sale debt securities, Fair Value, Total	39,753	17,078
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(104)	(123)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(928)	(186)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(1,032)	(309)
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	3,067	4,599
Available-for-sale debt securities, More than 12 Months, Fair Value	3,949	0
Available-for-sale debt securities, Fair Value, Total	7,016	4,599
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(19)	(22)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(166)	0
Available-for-sale debt securities, Gross Unrealized Loss, Total	(185)	(22)
Corporate debt securities \| Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	10,869	0
Available-for-sale debt securities, More than 12 Months, Fair Value	0	11,905
Available-for-sale debt securities, Fair Value, Total	10,869	11,905
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(381)	0
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	0	(746)
Available-for-sale debt securities, Gross Unrealized Loss, Total	(381)	(746)
Foreign bonds \| Investment grade
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	14,418	103,149
Available-for-sale debt securities, More than 12 Months, Fair Value	448,630	352,239
Available-for-sale debt securities, Fair Value, Total	463,048	455,388
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(40)	(1,325)
Available-for-sale debt securities, Gross Unrealized Loss, More than 12 Months	(26,290)	(17,687)
Available-for-sale debt securities, Gross Unrealized Loss, Total	$ (26,330)	(19,012)
Other securities
Fair Value and Gross Unrealized Losses
Other securities, Less than 12 Months, Fair Value		31,215
Other securities, Gross Unrealized Loss, Less than 12 Months		(545)
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
Other securities, Gross Unrealized Loss, Total		$ (545)

Equity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss

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