Investment Securities (Schedule of General Obligation Bonds) (Details) $ in Thousands	Jun. 30, 2016 USD ($) issuers	Dec. 31, 2015 USD ($) issuers	Jun. 30, 2015 USD ($)
general obligation bonds [Line Items]
General obligation bonds, number of issuers	636	712	773
General obligation bonds, average issue size, fair value	$ 1,037	$ 1,033	$ 910
General obligation bonds, amortized cost	642,763	712,035	694,291
General obligation bonds, fair value	$ 659,686	$ 735,350	$ 703,590
MICHIGAN
general obligation bonds [Line Items]
General obligation bonds, number of issuers	116	137	153
General obligation bonds, average issue size, fair value	$ 1,404	$ 1,381	$ 1,018
General obligation bonds, amortized cost	157,403	180,508	152,499
General obligation bonds, fair value	$ 162,897	$ 189,259	$ 155,712
Ohio
general obligation bonds [Line Items]
General obligation bonds, number of issuers	108	111	127
General obligation bonds, average issue size, fair value	$ 1,095	$ 1,091	$ 964
General obligation bonds, amortized cost	115,545	116,783	121,716
General obligation bonds, fair value	$ 118,300	$ 121,117	$ 122,474
Wisconsin
general obligation bonds [Line Items]
General obligation bonds, number of issuers	54	69	70
General obligation bonds, average issue size, fair value	$ 651	$ 673	$ 585
General obligation bonds, amortized cost	34,046	44,794	39,774
General obligation bonds, fair value	$ 35,166	$ 46,454	$ 40,950
ILLINOIS
general obligation bonds [Line Items]
General obligation bonds, number of issuers	52	55	60
General obligation bonds, average issue size, fair value	$ 1,783	$ 1,870	$ 1,784
General obligation bonds, amortized cost	90,772	99,524	105,581
General obligation bonds, fair value	$ 92,711	$ 102,867	$ 107,033
TEXAS
general obligation bonds [Line Items]
General obligation bonds, number of issuers	51	58	67
General obligation bonds, average issue size, fair value	$ 816	$ 807	$ 759
General obligation bonds, amortized cost	40,409	45,818	50,370
General obligation bonds, fair value	$ 41,629	$ 46,805	$ 50,877
PENNSYLVANIA
general obligation bonds [Line Items]
General obligation bonds, number of issuers	41	42	46
General obligation bonds, average issue size, fair value	$ 1,037	$ 1,020	$ 1,014
General obligation bonds, amortized cost	41,674	42,185	46,547
General obligation bonds, fair value	$ 42,529	$ 42,835	$ 46,666
NEW JERSEY
general obligation bonds [Line Items]
General obligation bonds, number of issuers	31	35	34
General obligation bonds, average issue size, fair value	$ 724	$ 725	$ 720
General obligation bonds, amortized cost	21,928	24,810	23,916
General obligation bonds, fair value	$ 22,441	$ 25,372	$ 24,472
WASHINGTON
general obligation bonds [Line Items]
General obligation bonds, number of issuers	27	29	30
General obligation bonds, average issue size, fair value	$ 968	$ 950	$ 939
General obligation bonds, amortized cost	25,586	27,080	27,783
General obligation bonds, fair value	$ 26,149	$ 27,548	$ 28,169
MINNESOTA
general obligation bonds [Line Items]
General obligation bonds, number of issuers	23	33	34
General obligation bonds, average issue size, fair value	$ 699	$ 667	$ 692
General obligation bonds, amortized cost	15,667	21,679	23,267
General obligation bonds, fair value	$ 16,076	$ 22,020	$ 23,540
NEW YORK
general obligation bonds [Line Items]
General obligation bonds, number of issuers	18	18	18
General obligation bonds, average issue size, fair value	$ 575	$ 635	$ 633
General obligation bonds, amortized cost	10,044	11,161	11,187
General obligation bonds, fair value	$ 10,346	$ 11,422	$ 11,392
MISSOURI
general obligation bonds [Line Items]
General obligation bonds, number of issuers	10	15	15
General obligation bonds, average issue size, fair value	$ 1,075	$ 1,078	$ 1,084
General obligation bonds, amortized cost	10,414	15,878	15,981
General obligation bonds, fair value	$ 10,754	$ 16,174	$ 16,265
Other states
general obligation bonds [Line Items]
General obligation bonds, number of issuers	105	110	119
General obligation bonds, average issue size, fair value	$ 768	$ 759	$ 639
General obligation bonds, amortized cost	79,275	81,815	75,670
General obligation bonds, fair value	$ 80,688	$ 83,477	$ 76,040