Investment Securities (Fair Value and Gross Unrealized Loss) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available for sale:
Fair value, less than 12 months	$ 189,270	$ 1,094,992
Unrealized losses, less than 12 Months	(754)	(9,387)
Fair value, 12 months or more	1,293,441	1,460,634
Unrealized losses, 12 Months or More	(13,544)	(37,440)
Fair value, total	1,482,711	2,555,626
Unrealized losses, total	(14,298)	(46,827)
Residential mortgage-backed securities
Available for sale:
Fair value, less than 12 months	187,800	1,083,137
Unrealized losses, less than 12 Months	(737)	(9,333)
Fair value, 12 months or more	1,287,279	1,454,550
Unrealized losses, 12 Months or More	(13,408)	(37,224)
Fair value, total	1,475,079	2,537,687
Unrealized losses, total	(14,145)	(46,557)
States and political subdivisions
Available for sale:
Fair value, less than 12 months	1,407	6,814
Unrealized losses, less than 12 Months	(5)	(19)
Fair value, 12 months or more	545	544
Unrealized losses, 12 Months or More	(3)	(6)
Fair value, total	1,952	7,358
Unrealized losses, total	(8)	(25)
Other
Available for sale:
Fair value, less than 12 months	63	5,041
Unrealized losses, less than 12 Months	(12)	(35)
Fair value, 12 months or more	5,617	5,540
Unrealized losses, 12 Months or More	(133)	(210)
Fair value, total	5,680	10,581
Unrealized losses, total	$ (145)	$ (245)