Investments - Schedule of Unrealized Loss on Investments (Details) $ in Thousands	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Less Than Twelve Months
Number of securities \| security	949	542
Gross unrealized losses	$ 10,588	$ 2,361
Fair value	$ 611,721	$ 349,028
Twelve Months or More
Number of securities \| security	205	214
Gross unrealized losses	$ 3,285	$ 2,394
Fair value	$ 104,305	$ 111,703
U.S. Government and Agency Securities
Less Than Twelve Months
Number of securities \| security	177	129
Gross unrealized losses	$ 3,252	$ 641
Fair value	$ 163,624	$ 103,328
Twelve Months or More
Number of securities \| security	120	123
Gross unrealized losses	$ 2,065	$ 1,552
Fair value	$ 69,114	$ 74,190
States, Municipalities and Political Subdivisions
Less Than Twelve Months
Number of securities \| security	159	106
Gross unrealized losses	$ 1,773	$ 734
Fair value	$ 133,508	$ 91,245
Twelve Months or More
Number of securities \| security	24	31
Gross unrealized losses	$ 435	$ 389
Fair value	$ 16,616	$ 19,718
Public Utilities
Less Than Twelve Months
Number of securities \| security	39	16
Gross unrealized losses	$ 381	$ 44
Fair value	$ 21,114	$ 7,052
Twelve Months or More
Number of securities \| security	5	5
Gross unrealized losses	$ 63	$ 41
Fair value	$ 994	$ 1,016
Corporate Securities
Less Than Twelve Months
Number of securities \| security	500	263
Gross unrealized losses	$ 4,406	$ 871
Fair value	$ 240,811	$ 134,755
Twelve Months or More
Number of securities \| security	52	52
Gross unrealized losses	$ 612	$ 338
Fair value	$ 17,182	$ 16,476
Asset-backed Securities
Less Than Twelve Months
Number of securities \| security	31	18
Gross unrealized losses	$ 57	$ 20
Fair value	$ 31,532	$ 11,682
Twelve Months or More
Number of securities \| security	0	0
Gross unrealized losses	$ 0	$ 0
Fair value	$ 0	$ 0
Redeemable Preferred Stocks
Less Than Twelve Months
Number of securities \| security	1	0
Gross unrealized losses	$ 4	$ 0
Fair value	$ 121	$ 0
Twelve Months or More
Number of securities \| security	3	3
Gross unrealized losses	$ 81	$ 74
Fair value	$ 302	$ 303
Fixed Maturities
Less Than Twelve Months
Number of securities \| security	909	532
Gross unrealized losses	$ 9,881	$ 2,310
Fair value	$ 592,722	$ 348,062
Twelve Months or More
Number of securities \| security	204	214
Gross unrealized losses	$ 3,256	$ 2,394
Fair value	$ 104,208	$ 111,703
Public Utilities
Less Than Twelve Months
Number of securities \| security	2
Gross unrealized losses	$ 34
Fair value	$ 416
Twelve Months or More
Number of securities \| security	0
Gross unrealized losses	$ 0
Fair value	$ 0
Common Stock
Less Than Twelve Months
Number of securities \| security	23	5
Gross unrealized losses	$ 202	$ 47
Fair value	$ 3,236	$ 748
Twelve Months or More
Number of securities \| security	1	0
Gross unrealized losses	$ 29	$ 0
Fair value	$ 97	$ 0
Nonredeemable Preferred Stocks
Less Than Twelve Months
Number of securities \| security	13	4
Gross unrealized losses	$ 17	$ 4
Fair value	$ 604	$ 87
Twelve Months or More
Number of securities \| security	0	0
Gross unrealized losses	$ 0	$ 0
Fair value	$ 0	$ 0
Equity Securities
Less Than Twelve Months
Number of securities \| security	40	10
Gross unrealized losses	$ 707	$ 51
Fair value	$ 18,999	$ 966
Twelve Months or More
Number of securities \| security	1	0
Gross unrealized losses	$ 29	$ 0
Fair value	$ 97	$ 0
Mutual Fund
Less Than Twelve Months
Number of securities \| security	2	1
Gross unrealized losses	$ 454	$ 0
Fair value	$ 14,743	$ 131
Twelve Months or More
Number of securities \| security	0	0
Gross unrealized losses	$ 0	$ 0
Fair value	$ 0	$ 0
Foreign Government
Less Than Twelve Months
Number of securities \| security	2
Gross unrealized losses	$ 8
Fair value	$ 2,012
Twelve Months or More
Number of securities \| security	0
Gross unrealized losses	$ 0
Fair value	$ 0

Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months [Abstract]

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Available For Sale Securities Continuous Unrealized Loss Position Twelve Months or More [Abstract]

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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