Investment Securities Investment Securities (Schedule of general obligation bonds) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	890	546
General obligation bonds, average issue size, fair value	$ 829	$ 994
General obligation bonds, amortized cost	739,490	523,203
General obligation bonds, fair value	737,946	543,265
MICHIGAN
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	166	127
General obligation bonds, average issue size, fair value	744	1,167
General obligation bonds, amortized cost	122,198	144,789
General obligation bonds, fair value	123,571	148,264
OHIO
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	154	42
General obligation bonds, average issue size, fair value	1,041	1,366
General obligation bonds, amortized cost	159,674	55,207
General obligation bonds, fair value	160,265	57,365
ILLINOIS
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	75	55
General obligation bonds, average issue size, fair value	1,314	970
General obligation bonds, amortized cost	96,863	51,932
General obligation bonds, fair value	98,521	53,348
TEXAS
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	69	53
General obligation bonds, average issue size, fair value	771	931
General obligation bonds, amortized cost	54,295	47,613
General obligation bonds, fair value	53,204	49,326
WISCONSIN
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	87	28
General obligation bonds, average issue size, fair value	645	1,096
General obligation bonds, amortized cost	54,921	28,572
General obligation bonds, fair value	56,152	30,687
PENNSYLVANIA
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	51	40
General obligation bonds, average issue size, fair value	891	719
General obligation bonds, amortized cost	48,319	27,348
General obligation bonds, fair value	45,451	28,756
MINNESOTA
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	45	33
General obligation bonds, average issue size, fair value	663	856
General obligation bonds, amortized cost	29,840	26,541
General obligation bonds, fair value	29,816	28,232
WASHINGTON
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	30	22
General obligation bonds, average issue size, fair value	930	1,233
General obligation bonds, amortized cost	28,393	26,104
General obligation bonds, fair value	27,906	27,130
NEW JERSEY
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	38	22
General obligation bonds, average issue size, fair value	722	907
General obligation bonds, amortized cost	27,101	18,976
General obligation bonds, fair value	27,440	19,958
MISSOURI
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	20	10
General obligation bonds, average issue size, fair value	969	1,482
General obligation bonds, amortized cost	19,253	13,809
General obligation bonds, fair value	19,382	14,815
NEW YORK
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	22	18
General obligation bonds, average issue size, fair value	585	639
General obligation bonds, amortized cost	13,064	11,027
General obligation bonds, fair value	12,878	11,507
CALIFORNIA
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	18	11
General obligation bonds, average issue size, fair value	599	930
General obligation bonds, amortized cost	10,651	9,734
General obligation bonds, fair value	10,788	10,231
Other states
general obligation bonds [Line Items]
General Obligation Bonds number of issuers	115	85
General obligation bonds, average issue size, fair value	631	742
General obligation bonds, amortized cost	74,918	61,551
General obligation bonds, fair value	$ 72,572	$ 63,646