Securities - Obligations by State (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Securities
Available for sale, fair value	$ 646,349	$ 759,811
Held to maturity, fair value	$ 209,003	$ 47,683
General obligation bonds
Securities
Number of issuers \| item	209	221
Amortized Cost	$ 153,800	$ 163,085
Fair Value	155,141	163,612
Average Exposure Per Issuer (Fair Value)	742	740
Amount of largest exposure to a single issuer	$ 3,400	$ 3,400
Percentage of the entity's stockholders equity in excess of aggregate book or market value	10.00%	10.00%
Percentage of underlying asset rated by at least a specified nationally recognized statistical rating organization	98.30%	98.40%
Minimum number of nationally recognized statistical rating organizations \| item	1	1
Percentage of underlying asset unrated	1.70%	1.60%
General obligation bonds \| Illinois
Securities
Number of issuers \| item	73	70
Amortized Cost	$ 65,270	$ 59,120
Fair Value	65,957	59,182
Average Exposure Per Issuer (Fair Value)	$ 904	$ 845
General obligation bonds \| Michigan
Securities
Number of issuers \| item	31	38
Amortized Cost	$ 18,011	$ 23,233
Fair Value	18,314	23,472
Average Exposure Per Issuer (Fair Value)	$ 591	$ 618
General obligation bonds \| Wisconsin
Securities
Number of issuers \| item	29	31
Amortized Cost	$ 17,435	$ 21,390
Fair Value	17,522	21,479
Average Exposure Per Issuer (Fair Value)	$ 604	$ 693
General obligation bonds \| Pennsylvania
Securities
Number of issuers \| item	7	10
Amortized Cost	$ 8,309	$ 10,242
Fair Value	8,333	10,235
Average Exposure Per Issuer (Fair Value)	$ 1,190	$ 1,023
General obligation bonds \| Texas
Securities
Number of issuers \| item	16	16
Amortized Cost	$ 10,121	$ 10,731
Fair Value	10,153	10,702
Average Exposure Per Issuer (Fair Value)	$ 635	$ 669
General obligation bonds \| Ohio
Securities
Number of issuers \| item	10	10
Amortized Cost	$ 10,964	$ 11,009
Fair Value	10,978	11,005
Average Exposure Per Issuer (Fair Value)	$ 1,098	$ 1,100
General obligation bonds \| Iowa
Securities
Number of issuers \| item		3
Amortized Cost		$ 5,332
Fair Value		5,345
Average Exposure Per Issuer (Fair Value)		$ 1,782
General obligation bonds \| Other
Securities
Number of issuers \| item	43	43
Amortized Cost	$ 23,690	$ 22,028
Fair Value	23,884	22,192
Average Exposure Per Issuer (Fair Value)	$ 555	$ 516
Revenue bonds
Securities
Number of issuers \| item	36	39
Amortized Cost	$ 30,988	$ 35,686
Fair Value	31,155	35,621
Average Exposure Per Issuer (Fair Value)	865	913
Amount of largest exposure to a single issuer	$ 3,600	$ 3,500
Percentage of the entity's stockholders equity in excess of aggregate book or market value	10.00%	10.00%
Percentage of bonds rated by nationally recognized statistical rating organization	98.70%	97.10%
Percentage of bonds unrated	1.30%	2.90%
Minimum number of nationally recognized statistical rating organizations \| item	1	1
Revenue bonds \| Indiana
Securities
Number of issuers \| item	10	10
Amortized Cost	$ 10,190	$ 11,207
Fair Value	10,318	11,244
Average Exposure Per Issuer (Fair Value)	$ 1,032	$ 1,124
Revenue bonds \| Illinois
Securities
Number of issuers \| item	7	7
Amortized Cost	$ 6,339	$ 7,321
Fair Value	6,384	7,275
Average Exposure Per Issuer (Fair Value)	$ 912	$ 1,039
Revenue bonds \| Other
Securities
Number of issuers \| item	19	22
Amortized Cost	$ 14,459	$ 17,158
Fair Value	14,453	17,102
Average Exposure Per Issuer (Fair Value)	761	777
Obligations of states and political subdivisions
Securities
Available for sale, fair value	143,353	154,938
Held to maturity, fair value	$ 42,943	$ 44,295
Number of issuers \| item	245	260
Total Capital for rated securities (as a percent)	100.00%
Total Capital for unrated securities (as a percent)	15.00%
Obligations of states and political subdivisions \| General obligation bonds
Securities
Fair Value	$ 155,100	$ 163,600
Number of states in which investment is held \| item	27	29
Number of states in which investment is held in excess of aggregate fair value \| item	6	7
Aggregate fair value	$ 5,000	$ 5,000
Obligations of states and political subdivisions \| Revenue bonds
Securities
Fair Value	$ 31,200	$ 35,600
Number of states in which investment is held \| item	15	16
Number of states in which investment is held in excess of aggregate fair value \| item	2	2
Aggregate fair value	$ 5,000	$ 5,000

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Percentage of bonds, which were unrated.

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Represents the threshold percentage of the entity's stockholders equity to be compared to aggregate book or market value of general obligation bonds.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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