Securities - Obligations by State (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Securities
Available for sale, fair value	$ 708,247	$ 759,811
Held to maturity, fair value	$ 282,488	$ 47,683
General obligation bonds
Securities
Number of issuers \| item	308	221
Amortized Cost	$ 222,974	$ 163,085
Fair Value	224,371	163,612
Average Exposure Per Issuer (Fair Value)	728	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	98.80%	98.40%
Minimum number of nationally recognized statistical rating organizations \| item	1	1
Percentage of underlying asset unrated	1.20%	1.60%
General obligation bonds \| Illinois
Securities
Number of issuers \| item	108	70
Amortized Cost	$ 94,616	$ 59,120
Fair Value	95,355	59,182
Average Exposure Per Issuer (Fair Value)	$ 883	$ 845
General obligation bonds \| Michigan
Securities
Number of issuers \| item	35	38
Amortized Cost	$ 20,543	$ 23,233
Fair Value	20,841	23,472
Average Exposure Per Issuer (Fair Value)	$ 595	$ 618
General obligation bonds \| Wisconsin
Securities
Number of issuers \| item	35	31
Amortized Cost	$ 24,000	$ 21,390
Fair Value	24,071	21,479
Average Exposure Per Issuer (Fair Value)	$ 688	$ 693
General obligation bonds \| Pennsylvania
Securities
Number of issuers \| item	20	10
Amortized Cost	$ 13,170	$ 10,242
Fair Value	13,196	10,235
Average Exposure Per Issuer (Fair Value)	$ 660	$ 1,023
General obligation bonds \| Texas
Securities
Number of issuers \| item	31	16
Amortized Cost	$ 18,946	$ 10,731
Fair Value	19,040	10,702
Average Exposure Per Issuer (Fair Value)	$ 614	$ 669
General obligation bonds \| Ohio
Securities
Number of issuers \| item	10	10
Amortized Cost	$ 10,692	$ 11,009
Fair Value	10,712	11,005
Average Exposure Per Issuer (Fair Value)	$ 1,071	$ 1,100
General obligation bonds \| New Jersey
Securities
Number of issuers \| item	13
Amortized Cost	$ 6,435
Fair Value	6,472
Average Exposure Per Issuer (Fair Value)	$ 498
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	56	43
Amortized Cost	$ 34,572	$ 22,028
Fair Value	34,684	22,192
Average Exposure Per Issuer (Fair Value)	$ 619	$ 516
Revenue bonds
Securities
Number of issuers \| item	42	39
Amortized Cost	$ 35,341	$ 35,686
Fair Value	35,574	35,621
Average Exposure Per Issuer (Fair Value)	847	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	98.60%	97.10%
Percentage of bonds unrated	1.40%	2.90%
Minimum number of nationally recognized statistical rating organizations \| item	1	1
Revenue bonds \| Indiana
Securities
Number of issuers \| item	8	10
Amortized Cost	$ 8,707	$ 11,207
Fair Value	8,832	11,244
Average Exposure Per Issuer (Fair Value)	$ 1,104	$ 1,124
Revenue bonds \| Illinois
Securities
Number of issuers \| item	9	7
Amortized Cost	$ 6,895	$ 7,321
Fair Value	6,953	7,275
Average Exposure Per Issuer (Fair Value)	$ 773	$ 1,039
Revenue bonds \| Other
Securities
Number of issuers \| item	25	22
Amortized Cost	$ 19,739	$ 17,158
Fair Value	19,789	17,102
Average Exposure Per Issuer (Fair Value)	792	777
Obligations of states and political subdivisions
Securities
Available for sale, fair value	217,108	154,938
Held to maturity, fair value	$ 42,837	$ 44,295
Number of issuers \| item	350	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	$ 224,400	$ 163,600
Number of states in which investment is held \| item	30	29
Number of states in which investment is held in excess of aggregate fair value \| item	7	7
Aggregate fair value	$ 5,000	$ 5,000
Obligations of states and political subdivisions \| Revenue bonds
Securities
Fair Value	$ 35,500	$ 35,600
Number of states in which investment is held \| item	18	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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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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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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