Segment Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) location	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment location	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of business segments | segment			2
Net premiums earned	$ 259,563	$ 253,487	$ 513,435	$ 494,785
Investment income, net of investment expenses	24,610	24,507	49,645	46,731
Net realized investment gains	2,680	1,596	6,634	3,651
Other income	126	183	324	291
Total revenues	286,979	279,773	570,038	545,458
Net income	2,958	3,114	22,894	25,541
Assets	4,197,412	4,105,838	4,197,412	4,105,838	$ 4,054,758
Invested assets	$ 3,328,928	3,271,406	$ 3,328,928	3,271,406	$ 3,265,660
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Number of domestic locations | location	6		6
Total revenues	$ 258,487	244,866	$ 509,765	476,200
Net income	109	3,434	18,693	25,454
Assets	2,595,147	2,459,526	2,595,147	2,459,526
Invested assets	1,836,736	1,723,641	1,836,736	1,723,641
Life Insurance
Segment Reporting Information [Line Items]
Total revenues	28,492	34,907	60,273	69,258
Net income	2,849	(320)	4,201	87
Assets	1,602,265	1,646,312	1,602,265	1,646,312
Invested assets	1,492,192	1,547,765	1,492,192	1,547,765
Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	259,563	253,487	513,435	494,785
Investment income, net of investment expenses	24,634	24,544	49,689	46,805
Net realized investment gains	2,680	1,596	6,634	3,651
Other income	126	183	324	291
Total revenues	287,003	279,810	570,082	545,532
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net premiums earned	245,222	232,282	481,666	452,507
Investment income, net of investment expenses	12,208	11,655	24,813	21,064
Net realized investment gains	1,081	966	3,330	2,703
Other income	0	0	0	0
Total revenues	258,511	244,903	509,809	476,274
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned	14,341	21,205	31,769	42,278
Investment income, net of investment expenses	12,426	12,889	24,876	25,741
Net realized investment gains	1,599	630	3,304	948
Other income	126	183	324	291
Total revenues	28,492	34,907	60,273	69,258
Intersegment eliminations
Segment Reporting Information [Line Items]
Total revenues	(24)	(37)	(44)	(74)
Intersegment eliminations | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total revenues	(24)	(37)	(44)	(74)
Intersegment eliminations | Life Insurance
Segment Reporting Information [Line Items]
Total revenues	$ 0	$ 0	$ 0	$ 0