Investment Securities (Schedule of General Obligation Bonds) (Details) $ in Thousands	Dec. 31, 2015 USD ($) issuers	Dec. 31, 2014 USD ($) issuers
general obligation bonds [Line Items]
Number of Issuers | issuers	712	825
Average Issue Size, Fair Value	$ 1,033	$ 917
Amortized Cost	712,035	737,673
Fair Value	$ 735,350	$ 756,315
MICHIGAN
general obligation bonds [Line Items]
Number of Issuers | issuers	137	169
Average Issue Size, Fair Value	$ 1,381	$ 842
Amortized Cost	180,508	138,325
Fair Value	$ 189,259	$ 142,292
OHIO
general obligation bonds [Line Items]
Number of Issuers | issuers	111	137
Average Issue Size, Fair Value	$ 1,091	$ 979
Amortized Cost	116,783	130,741
Fair Value	$ 121,117	$ 134,127
ILLINOIS
general obligation bonds [Line Items]
Number of Issuers | issuers	55	66
Average Issue Size, Fair Value	$ 1,870	$ 1,897
Amortized Cost	99,524	121,560
Fair Value	$ 102,867	$ 125,169
TEXAS
general obligation bonds [Line Items]
Number of Issuers | issuers	58	64
Average Issue Size, Fair Value	$ 807	$ 801
Amortized Cost	45,818	50,307
Fair Value	$ 46,805	$ 51,293
WISCONSIN
general obligation bonds [Line Items]
Number of Issuers | issuers	69	77
Average Issue Size, Fair Value	$ 673	$ 841
Amortized Cost	44,794	62,543
Fair Value	$ 46,454	$ 64,776
PENNSYLVANIA
general obligation bonds [Line Items]
Number of Issuers | issuers	42	45
Average Issue Size, Fair Value	$ 1,020	$ 1,000
Amortized Cost	42,185	44,443
Fair Value	$ 42,835	$ 45,006
WASHINGTON
general obligation bonds [Line Items]
Number of Issuers | issuers	29	30
Average Issue Size, Fair Value	$ 950	$ 952
Amortized Cost	27,080	27,987
Fair Value	$ 27,548	$ 28,558
NEW JERSEY
general obligation bonds [Line Items]
Number of Issuers | issuers	35	37
Average Issue Size, Fair Value	$ 725	$ 746
Amortized Cost	24,810	26,755
Fair Value	$ 25,372	$ 27,612
MINNESOTA
general obligation bonds [Line Items]
Number of Issuers | issuers	33	42
Average Issue Size, Fair Value	$ 667	$ 674
Amortized Cost	21,679	27,740
Fair Value	$ 22,020	$ 28,326
MISSOURI
general obligation bonds [Line Items]
Number of Issuers | issuers	15	19
Average Issue Size, Fair Value	$ 1,078	$ 1,011
Amortized Cost	15,878	18,764
Fair Value	$ 16,174	$ 19,207
NEW YORK
general obligation bonds [Line Items]
Number of Issuers | issuers	18	19
Average Issue Size, Fair Value	$ 635	$ 628
Amortized Cost	11,161	11,659
Fair Value	$ 11,422	$ 11,929
Other states
general obligation bonds [Line Items]
Number of Issuers | issuers	110	120
Average Issue Size, Fair Value	$ 759	$ 650
Amortized Cost	81,815	76,849
Fair Value	$ 83,477	$ 78,020