Investment Securities (Fair Value and Gross Unrealized Loss) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available for sale:
Fair value, less than 12 months	$ 813,131	$ 1,094,992
Unrealized losses, less than 12 Months	(4,010)	(9,387)
Fair value, 12 months or more	418,620	1,460,634
Unrealized losses, 12 Months or More	(6,919)	(37,440)
Fair value, total	1,231,751	2,555,626
Unrealized losses, total	(10,929)	(46,827)
Residential mortgage-backed securities
Available for sale:
Fair value, less than 12 months	812,590	1,083,137
Unrealized losses, less than 12 Months	(4,009)	(9,333)
Fair value, 12 months or more	412,993	1,454,550
Unrealized losses, 12 Months or More	(6,796)	(37,224)
Fair value, total	1,225,583	2,537,687
Unrealized losses, total	(10,805)	(46,557)
Obligations of states and political subdivisions
Available for sale:
Fair value, less than 12 months	541	6,814
Unrealized losses, less than 12 Months	(1)	(19)
Fair value, 12 months or more		544
Unrealized losses, 12 Months or More		(6)
Fair value, total	541	7,358
Unrealized losses, total	(1)	(25)
Equity securities
Available for sale:
Fair value, less than 12 months		5,041
Unrealized losses, less than 12 Months		(35)
Fair value, 12 months or more	5,627	5,540
Unrealized losses, 12 Months or More	(123)	(210)
Fair value, total	5,627	10,581
Unrealized losses, total	$ (123)	$ (245)