Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment securities available for sale:
Fair Value, Less than 12 months	$ 911,980	$ 1,173,274
Unrealized Losses, Less than 12 months	3,024	2,203
Fair Value, 12 months or more	300,157	832,420
Unrealized Losses, Greater than 12 months	3,895	10,043
Fair Value, Total	1,212,137	2,005,694
Unrealized Losses, Total	6,919	12,246
U. S. Treasury
Investment securities available for sale:
Fair Value, Less than 12 months		338,612
Unrealized Losses, Less than 12 months		151
Fair Value, 12 months or more		1,015
Unrealized Losses, Greater than 12 months		1
Fair Value, Total		339,627
Unrealized Losses, Total	0	152
Government Agency
Investment securities available for sale:
Fair Value, Less than 12 months		261,288
Unrealized Losses, Less than 12 months		247
Fair Value, 12 months or more		0
Unrealized Losses, Greater than 12 months		0
Fair Value, Total		261,288
Unrealized Losses, Total	0	247
Mortgage Backed Securities
Investment securities available for sale:
Fair Value, Less than 12 months	911,980	573,374
Unrealized Losses, Less than 12 months	3,024	1,805
Fair Value, 12 months or more	300,157	831,405
Unrealized Losses, Greater than 12 months	3,895	10,042
Fair Value, Total	1,212,137	1,404,779
Unrealized Losses, Total	$ 6,919	11,847
State, County and Municipal
Investment securities available for sale:
Unrealized Losses, Total		$ 0