Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investment securities available for sale:
Fair Value, Less than 12 months	$ 1,962,911	$ 1,173,274
Unrealized Losses, Less than 12 months	13,121	2,203
Fair Value, 12 months or more	296,170	832,420
Unrealized Losses, Greater than 12 months	4,783	10,043
Fair Value, Total	2,259,081	2,005,694
Unrealized Losses, Total	17,904	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	90,567	261,288
Unrealized Losses, Less than 12 months	385	247
Fair Value, 12 months or more	0	0
Unrealized Losses, Greater than 12 months	0	0
Fair Value, Total	90,567	261,288
Unrealized Losses, Total	385	247
Mortgage Backed Securities, Other [Member]
Investment securities available for sale:
Fair Value, Less than 12 months	1,872,344	573,374
Unrealized Losses, Less than 12 months	12,736	1,805
Fair Value, 12 months or more	296,170	831,405
Unrealized Losses, Greater than 12 months	4,783	10,042
Fair Value, Total	2,168,514	1,404,779
Unrealized Losses, Total	$ 17,519	11,847
State, County and Municipal
Investment securities available for sale:
Unrealized Losses, Total		$ 0