Securities (Debt securities available for sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months	$ 1,232,833
12 months or longer	812,622
Total	2,045,455
Less than 12 months		$ 948,929
12 months or longer		559,158
Total		1,508,087
Unrealized Losses
Less than 12 months	(23,890)
12 months or longer	(46,452)
Total	(70,342)
Less than 12 months		(12,238)
12 months or longer		(30,101)
Total		(42,339)
U.S. government corporations and agencies
Fair Value
Less than 12 months	22,392
12 months or longer	4,605
Total	26,997
Less than 12 months		4,819
12 months or longer		0
Total		4,819
Unrealized Losses
Less than 12 months	(39)
12 months or longer	(76)
Total	(115)
Less than 12 months		(38)
12 months or longer		0
Total		(38)
Mortgage and asset-backed securities
Fair Value
Less than 12 months	1,007,078
12 months or longer	668,623
Total	1,675,701
Less than 12 months		851,070
12 months or longer		399,978
Total		1,251,048
Unrealized Losses
Less than 12 months	(18,299)
12 months or longer	(38,026)
Total	(56,325)
Less than 12 months		(11,533)
12 months or longer		(26,054)
Total		(37,587)
Obligations of states and political subdivisions
Fair Value
Less than 12 months	203,363
12 months or longer	139,394
Total	342,757
Less than 12 months		93,040
12 months or longer		159,180
Total		252,220
Unrealized Losses
Less than 12 months	(5,552)
12 months or longer	(8,350)
Total	$ (13,902)
Less than 12 months		(667)
12 months or longer		(4,047)
Total		$ (4,714)