Securities (Available for Sale Securities Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Less than 12 months	$ 793,718	$ 948,929
12 months or longer	1,067,809	559,158
Total	1,861,527	1,508,087
Unrealized Losses
Less than 12 months	(9,533)	(12,238)
12 months or longer	(37,040)	(30,101)
Total	(46,573)	(42,339)
U.S. government corporations and agencies
Fair Value
Less than 12 months	24,902	4,819
12 months or longer	4,577	0
Total	29,479	4,819
Unrealized Losses
Less than 12 months	(83)	(38)
12 months or longer	(44)	0
Total	(127)	(38)
Mortgage and asset-backed securities
Fair Value
Less than 12 months	733,826	851,070
12 months or longer	805,089	399,978
Total	1,538,915	1,251,048
Unrealized Losses
Less than 12 months	(9,060)	(11,533)
12 months or longer	(29,340)	(26,054)
Total	(38,400)	(37,587)
Obligations of states and political subdivisions
Fair Value
Less than 12 months	34,990	93,040
12 months or longer	258,143	159,180
Total	293,133	252,220
Unrealized Losses
Less than 12 months	(390)	(667)
12 months or longer	(7,656)	(4,047)
Total	$ (8,046)	$ (4,714)