Investment Securities (Schedule of general obligation bonds) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	825	886
General obligation bonds, average issue size, fair value	$ 917	$ 831
General obligation bonds, amortized cost	737,673	737,568
General obligation bonds, fair value	756,315	735,940
MICHIGAN
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	169	166
General obligation bonds, average issue size, fair value	842	744
General obligation bonds, amortized cost	138,325	122,198
General obligation bonds, fair value	142,292	123,571
OHIO
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	137	152
General obligation bonds, average issue size, fair value	979	1,048
General obligation bonds, amortized cost	130,741	158,641
General obligation bonds, fair value	134,127	159,232
ILLINOIS
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	66	75
General obligation bonds, average issue size, fair value	1,897	1,314
General obligation bonds, amortized cost	121,560	96,863
General obligation bonds, fair value	125,169	98,521
WISCONSIN
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	77	86
General obligation bonds, average issue size, fair value	841	648
General obligation bonds, amortized cost	62,543	54,516
General obligation bonds, fair value	64,776	55,747
TEXAS
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	64	69
General obligation bonds, average issue size, fair value	801	771
General obligation bonds, amortized cost	50,307	54,295
General obligation bonds, fair value	51,293	53,204
PENNSYLVANIA
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	45	50
General obligation bonds, average issue size, fair value	1,000	898
General obligation bonds, amortized cost	44,443	47,835
General obligation bonds, fair value	45,006	44,883
MINNESOTA
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	42	45
General obligation bonds, average issue size, fair value	674	663
General obligation bonds, amortized cost	27,740	29,840
General obligation bonds, fair value	28,326	29,816
WASHINGTON
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	30	30
General obligation bonds, average issue size, fair value	952	930
General obligation bonds, amortized cost	27,987	28,393
General obligation bonds, fair value	28,558	27,906
NEW JERSEY
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	37	38
General obligation bonds, average issue size, fair value	746	722
General obligation bonds, amortized cost	26,755	27,101
General obligation bonds, fair value	27,612	27,440
MISSOURI
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	19	20
General obligation bonds, average issue size, fair value	1,011	969
General obligation bonds, amortized cost	18,764	19,253
General obligation bonds, fair value	19,207	19,382
NEW YORK
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	19	22
General obligation bonds, average issue size, fair value	628	585
General obligation bonds, amortized cost	11,659	13,064
General obligation bonds, fair value	11,929	12,878
CALIFORNIA
general obligation bonds [Line Items]
General Obligation Bonds number of issuers		18
General obligation bonds, average issue size, fair value		599
General obligation bonds, amortized cost		10,651
General obligation bonds, fair value		10,788
Other states
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	120	115
General obligation bonds, average issue size, fair value	650	631
General obligation bonds, amortized cost	76,849	74,918
General obligation bonds, fair value	$ 78,020	$ 72,572