Securities (Available for Sale Securities Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 months	$ 1,619,266	$ 793,718
12 months or longer	241,232	1,067,809
Total	1,860,498	1,861,527
Unrealized Losses
Less than 12 months	(23,588)	(9,533)
12 months or longer	(4,814)	(37,040)
Total	(28,402)	(46,573)
U.S. government corporations and agencies
Fair Value
Less than 12 months	0	24,902
12 months or longer	0	4,577
Total	0	29,479
Unrealized Losses
Less than 12 months	0	(83)
12 months or longer	0	(44)
Total	0	(127)
Mortgage and asset-backed securities
Fair Value
Less than 12 months	1,231,732	733,826
12 months or longer	241,232	805,089
Total	1,472,964	1,538,915
Unrealized Losses
Less than 12 months	(14,189)	(9,060)
12 months or longer	(4,814)	(29,340)
Total	(19,003)	(38,400)
Obligations of states and political subdivisions
Fair Value
Less than 12 months	387,534	34,990
12 months or longer	0	258,143
Total	387,534	293,133
Unrealized Losses
Less than 12 months	(9,399)	(390)
12 months or longer	0	(7,656)
Total	$ (9,399)	$ (8,046)