Securities - Obligations by State (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Securities
Available for sale, fair value	$ 871,338	$ 872,682
Held to maturity, fair value	$ 496,715	$ 441,052
General obligation bonds
Securities
Number of Issuers \| item	345	381
Amortized Cost	$ 241,284	$ 271,499
Fair Value	239,171	271,659
Average Exposure Per Issuer (Fair Value)	693	713
Amount of largest exposure to a single issuer	$ 3,900	$ 4,000
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 rating organization	99.30%	99.30%
Percentage of bonds unrated	0.70%	0.70%
Minimum number of nationally recognized rating organizations \| item	1	1
General obligation bonds \| Illinois
Securities
Number of Issuers \| item	89	97
Amortized Cost	$ 88,096	$ 95,340
Fair Value	87,384	95,344
Average Exposure Per Issuer (Fair Value)	$ 982	$ 983
General obligation bonds \| Wisconsin
Securities
Number of Issuers \| item	29	41
Amortized Cost	$ 19,713	$ 27,852
Fair Value	19,511	27,809
Average Exposure Per Issuer (Fair Value)	$ 673	$ 678
General obligation bonds \| Texas
Securities
Number of Issuers \| item	45	46
Amortized Cost	$ 25,639	$ 27,485
Fair Value	25,287	27,514
Average Exposure Per Issuer (Fair Value)	$ 562	$ 598
General obligation bonds \| Michigan
Securities
Number of Issuers \| item	27	34
Amortized Cost	$ 14,243	$ 19,641
Fair Value	14,316	19,849
Average Exposure Per Issuer (Fair Value)	$ 530	$ 584
General obligation bonds \| Ohio
Securities
Number of Issuers \| item	20	20
Amortized Cost	$ 15,068	$ 15,172
Fair Value	14,969	15,162
Average Exposure Per Issuer (Fair Value)	$ 748	$ 758
General obligation bonds \| Pennsylvania
Securities
Number of Issuers \| item	18	18
Amortized Cost	$ 10,995	$ 12,189
Fair Value	10,970	12,174
Average Exposure Per Issuer (Fair Value)	$ 609	$ 676
General obligation bonds \| New Jersey
Securities
Number of Issuers \| item	14	15
Amortized Cost	$ 6,319	$ 7,755
Fair Value	6,277	7,760
Average Exposure Per Issuer (Fair Value)	$ 450	$ 517
General obligation bonds \| Missouri
Securities
Number of Issuers \| item	9	10
Amortized Cost	$ 5,582	$ 5,759
Fair Value	5,533	5,747
Average Exposure Per Issuer (Fair Value)	$ 615	$ 575
General obligation bonds \| Minnesota
Securities
Number of Issuers \| item		8
Amortized Cost		$ 5,657
Fair Value		5,667
Average Exposure Per Issuer (Fair Value)		$ 708
General obligation bonds \| Other
Securities
Number of Issuers \| item	94	92
Amortized Cost	$ 55,629	$ 54,649
Fair Value	54,924	54,633
Average Exposure Per Issuer (Fair Value)	$ 584	$ 594
Revenue bonds
Securities
Number of Issuers \| item	60	65
Amortized Cost	$ 46,678	$ 50,017
Fair Value	46,249	50,004
Average Exposure Per Issuer (Fair Value)	771	769
Amount of largest exposure to a single issuer	$ 3,500	$ 3,600
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 rating organization	100.00%	99.40%
Percentage of bonds unrated		0.60%
Minimum number of nationally recognized rating organizations \| item	1	1
Revenue bonds \| Indiana
Securities
Number of Issuers \| item	14	14
Amortized Cost	$ 11,953	$ 12,001
Fair Value	11,894	12,054
Average Exposure Per Issuer (Fair Value)	$ 850	$ 861
Revenue bonds \| Illinois
Securities
Number of Issuers \| item	6	7
Amortized Cost	$ 5,622	$ 6,477
Fair Value	5,561	6,456
Average Exposure Per Issuer (Fair Value)	$ 927	$ 922
Revenue bonds \| Missouri
Securities
Number of Issuers \| item	5	6
Amortized Cost	$ 7,048	$ 7,376
Fair Value	6,987	7,336
Average Exposure Per Issuer (Fair Value)	$ 1,397	$ 1,223
Revenue bonds \| Other
Securities
Number of Issuers \| item	35	38
Amortized Cost	$ 22,055	$ 24,163
Fair Value	21,807	24,158
Average Exposure Per Issuer (Fair Value)	623	636
Obligations of states and political subdivisions
Securities
Available for sale, fair value	246,654	280,199
Held to maturity, fair value	$ 38,766	$ 41,464
Number of Issuers \| item	405	446
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	$ 239,200	$ 271,700
Number of states in which investment is held \| item	35	36
Number of states in which investment is held in excess of aggregate fair value \| item	8	9
Aggregate fair value	$ 5,000	$ 5,000
Obligations of states and political subdivisions \| Revenue bonds
Securities
Fair Value	$ 46,200	$ 50,000
Number of states in which investment is held \| item	21	22
Number of states in which investment is held in excess of aggregate fair value \| item	3	3
Aggregate fair value	$ 5,000	$ 5,000

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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 debt 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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