Investment Securities (Schedule of General Obligation Bonds) (Details) $ in Thousands	Mar. 31, 2016 USD ($) issuers	Dec. 31, 2015 USD ($) issuers	Mar. 31, 2015 USD ($)
general obligation bonds [Line Items]
General obligation bonds, number of issuers	676	712	802
General obligation bonds, average issue size, fair value	$ 1,035	$ 1,033	$ 896,000
General obligation bonds, amortized cost	681,337	712,035	705,542,000
General obligation bonds, fair value	$ 699,549	$ 735,350	$ 718,992,000
MICHIGAN
general obligation bonds [Line Items]
General obligation bonds, number of issuers	137	137	136
General obligation bonds, average issue size, fair value	$ 1,364	$ 1,381	$ 925,000
General obligation bonds, amortized cost	180,322	180,508	124,908,000
General obligation bonds, fair value	$ 186,935	$ 189,259	$ 125,864,000
OHIO
general obligation bonds [Line Items]
General obligation bonds, number of issuers	110	111	164
General obligation bonds, average issue size, fair value	$ 1,098	$ 1,091	$ 857,000
General obligation bonds, amortized cost	117,643	116,783	138,091,000
General obligation bonds, fair value	$ 120,829	$ 121,117	$ 140,620,000
ILLINOIS
general obligation bonds [Line Items]
General obligation bonds, number of issuers	61	55	62
General obligation bonds, average issue size, fair value	$ 615	$ 1,870	$ 1,841,000
General obligation bonds, amortized cost	36,369	99,524	111,649,000
General obligation bonds, fair value	$ 37,528	$ 102,867	$ 114,150,000
TEXAS
general obligation bonds [Line Items]
General obligation bonds, number of issuers	53	58	73
General obligation bonds, average issue size, fair value	$ 1,840	$ 807	$ 760,000
General obligation bonds, amortized cost	95,228	45,818	53,877,000
General obligation bonds, fair value	$ 97,511	$ 46,805	$ 55,497,000
WISCONSIN
general obligation bonds [Line Items]
General obligation bonds, number of issuers	53	69	63
General obligation bonds, average issue size, fair value	$ 802	$ 673	$ 784,000
General obligation bonds, amortized cost	41,356	44,794	48,388,000
General obligation bonds, fair value	$ 42,496	$ 46,454	$ 49,396,000
PENNSYLVANIA
general obligation bonds [Line Items]
General obligation bonds, number of issuers	41	42	46
General obligation bonds, average issue size, fair value	$ 1,035	$ 1,020	$ 1,018,000
General obligation bonds, amortized cost	41,680	42,185	46,085,000
General obligation bonds, fair value	$ 42,448	$ 42,835	$ 46,816,000
WASHINGTON
general obligation bonds [Line Items]
General obligation bonds, number of issuers	33	29	31
General obligation bonds, average issue size, fair value	$ 736	$ 950	$ 930,000
General obligation bonds, amortized cost	23,746	27,080	28,194,000
General obligation bonds, fair value	$ 24,285	$ 27,548	$ 28,836,000
NEW JERSEY
general obligation bonds [Line Items]
General obligation bonds, number of issuers	29	35	37
General obligation bonds, average issue size, fair value	$ 948	$ 725	$ 747,000
General obligation bonds, amortized cost	26,977	24,810	26,724,000
General obligation bonds, fair value	$ 27,497	$ 25,372	$ 27,623,000
MINNESOTA
general obligation bonds [Line Items]
General obligation bonds, number of issuers	23	33	35
General obligation bonds, average issue size, fair value	$ 699	$ 667	$ 697,000
General obligation bonds, amortized cost	15,713	21,679	23,821,000
General obligation bonds, fair value	$ 16,069	$ 22,020	$ 24,405,000
MISSOURI
general obligation bonds [Line Items]
General obligation bonds, number of issuers	18	15	15
General obligation bonds, average issue size, fair value	$ 617	$ 1,078	$ 1,098,000
General obligation bonds, amortized cost	10,819	15,878	16,032,000
General obligation bonds, fair value	$ 11,104	$ 16,174	$ 16,472,000
NEW YORK
general obligation bonds [Line Items]
General obligation bonds, number of issuers	11	18	19
General obligation bonds, average issue size, fair value	$ 1,086	$ 635	$ 629,000
General obligation bonds, amortized cost	11,637	11,161	11,646,000
General obligation bonds, fair value	$ 11,948	$ 11,422	$ 11,948,000
Other states
general obligation bonds [Line Items]
General obligation bonds, number of issuers	107	110	121
General obligation bonds, average issue size, fair value	$ 756	$ 759	$ 639,000
General obligation bonds, amortized cost	79,847	81,815	76,127,000
General obligation bonds, fair value	$ 80,899	$ 83,477	$ 77,365,000