Securities (Continuous Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	$ 109,279	$ 163,128
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(552)	(1,518)
Available-for-sale debt securities, 12 Months or More, Fair Value	1,972,331	2,087,534
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(42,354)	(69,249)
Available-for-sale debt securities Fair Value, Total	2,081,610	2,250,662
Available-for-sale debt securities, Gross Unrealized Loss, Total	(42,906)	(70,767)
U.S. Treasury securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	0	0
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	0	0
Available-for-sale debt securities, 12 Months or More, Fair Value	520,440	516,520
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(8,543)	(12,899)
Available-for-sale debt securities Fair Value, Total	520,440	516,520
Available-for-sale debt securities, Gross Unrealized Loss, Total	(8,543)	(12,899)
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	0	22,755
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	0	(238)
Available-for-sale debt securities, 12 Months or More, Fair Value	153,149	159,814
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(1,313)	(2,456)
Available-for-sale debt securities Fair Value, Total	153,149	182,569
Available-for-sale debt securities, Gross Unrealized Loss, Total	(1,313)	(2,694)
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	31,872	26,886
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(12)	(245)
Available-for-sale debt securities, 12 Months or More, Fair Value	268,426	274,666
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(9,540)	(12,449)
Available-for-sale debt securities Fair Value, Total	300,298	301,552
Available-for-sale debt securities, Gross Unrealized Loss, Total	(9,552)	(12,694)
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	38,927	75,675
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(231)	(491)
Available-for-sale debt securities, 12 Months or More, Fair Value	543,638	653,660
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(6,091)	(14,061)
Available-for-sale debt securities Fair Value, Total	582,565	729,335
Available-for-sale debt securities, Gross Unrealized Loss, Total	(6,322)	(14,552)
Municipal securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	4,895	9,458
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(5)	(104)
Available-for-sale debt securities, 12 Months or More, Fair Value	21,660	30,295
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(184)	(928)
Available-for-sale debt securities Fair Value, Total	26,555	39,753
Available-for-sale debt securities, Gross Unrealized Loss, Total	(189)	(1,032)
Non-agency residential mortgage-backed securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	0	3,067
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	0	(19)
Available-for-sale debt securities, 12 Months or More, Fair Value	6,695	3,949
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(84)	(166)
Available-for-sale debt securities Fair Value, Total	6,695	7,016
Available-for-sale debt securities, Gross Unrealized Loss, Total	(84)	(185)
Corporate debt securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	9,838	10,869
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(163)	(381)
Available-for-sale debt securities, 12 Months or More, Fair Value	0	0
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	0	0
Available-for-sale debt securities Fair Value, Total	9,838	10,869
Available-for-sale debt securities, Gross Unrealized Loss, Total	(163)	(381)
Foreign bonds
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	11,202	14,418
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(59)	(40)
Available-for-sale debt securities, 12 Months or More, Fair Value	458,323	448,630
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	(16,599)	(26,290)
Available-for-sale debt securities Fair Value, Total	469,525	463,048
Available-for-sale debt securities, Gross Unrealized Loss, Total	(16,658)	$ (26,330)
Asset-backed securities
Fair Value and Gross Unrealized Losses
Available-for-sale debt securities, Less than 12 Months, Fair Value	12,545
Available-for-sale debt securities, Gross Unrealized Loss, Less than 12 Months	(82)
Available-for-sale debt securities, 12 Months or More, Fair Value	0
Available-for-sale debt securities, Gross Unrealized Loss, 12 Months or More	0
Available-for-sale debt securities Fair Value, Total	12,545
Available-for-sale debt securities, Gross Unrealized Loss, Total	$ (82)