Reserve for Losses and Loss Adjustment Expenses - IBNR Liabilities and Claims Frequency (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Claim claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 672,102	$ 600,276	$ 555,377
Casualty Reinsurance | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	63,357
IBNR	723
Casualty Reinsurance | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	105,333
IBNR	1,184
Casualty Reinsurance | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	146,045
IBNR	2,025
Casualty Reinsurance | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	146,760
IBNR	2,631
Casualty Reinsurance | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	126,160
IBNR	3,487
Casualty Reinsurance | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	114,517
IBNR	9,957
Casualty Reinsurance | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	108,222
IBNR	18,720
Casualty Reinsurance | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	129,800
IBNR	36,167
Casualty Reinsurance | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	119,098
IBNR	29,472
Casualty Reinsurance | Accident year 2019
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	86,022
IBNR	68,499
Fronting and Programs | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	52
IBNR	$ 0
Cumulative Number of Reported Claims | Claim	22
Fronting and Programs | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 3,083
IBNR	$ 147
Cumulative Number of Reported Claims | Claim	857
Fronting and Programs | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 8,384
IBNR	$ 432
Cumulative Number of Reported Claims | Claim	1,362
Fronting and Programs | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 15,522
IBNR	$ 1,861
Cumulative Number of Reported Claims | Claim	2,807
Fronting and Programs | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 25,201
IBNR	$ 6,751
Cumulative Number of Reported Claims | Claim	6,706
Fronting and Programs | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 20,677
IBNR	$ 10,142
Cumulative Number of Reported Claims | Claim	7,158
Fronting and Programs | Specialty Admitted Insurance | Accident year 2019
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 18,832
IBNR	$ 10,961
Cumulative Number of Reported Claims | Claim	5,695
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Cumulative Number of Reported Claims | claim	4
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 27,099
IBNR	$ 698
Cumulative Number of Reported Claims | Claim	1,604
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 36,593
IBNR	$ 1,879
Cumulative Number of Reported Claims | Claim	1,814
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 28,186
IBNR	$ 1,090
Cumulative Number of Reported Claims | Claim	1,323
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 10,909
IBNR	$ 572
Cumulative Number of Reported Claims | Claim	540
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 12,704
IBNR	$ 423
Cumulative Number of Reported Claims | Claim	850
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 17,626
IBNR	$ 1,440
Cumulative Number of Reported Claims | Claim	975
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 15,800
IBNR	$ 3,245
Cumulative Number of Reported Claims | Claim	836
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 19,779
IBNR	$ 3,888
Cumulative Number of Reported Claims | Claim	1,092
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 16,288
IBNR	$ 3,495
Cumulative Number of Reported Claims | Claim	1,230
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2019
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 20,253
IBNR	$ 2,965
Cumulative Number of Reported Claims | Claim	1,482
Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 1,277
IBNR	$ 1
Cumulative Number of Reported Claims | Claim	54
Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 18,303
IBNR	$ 150
Cumulative Number of Reported Claims | Claim	7,763
Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 36,636
IBNR	$ 337
Cumulative Number of Reported Claims | Claim	41,757
Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 147,122
IBNR	$ 5,258
Cumulative Number of Reported Claims | Claim	89,003
Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 272,421
IBNR	$ 11,473
Cumulative Number of Reported Claims | Claim	133,654
Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 230,220
IBNR	$ 34,896
Cumulative Number of Reported Claims | Claim	95,889
Commercial Auto | Excess and Surplus Lines | Accident year 2019
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 262,306
IBNR	$ 167,476
Cumulative Number of Reported Claims | Claim	67,380
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Cumulative Number of Reported Claims | claim	60
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 69,594
IBNR	$ 1,347
Cumulative Number of Reported Claims | Claim	1,429
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 105,173
IBNR	$ 2,380
Cumulative Number of Reported Claims | Claim	1,480
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 103,061
IBNR	$ 2,042
Cumulative Number of Reported Claims | Claim	1,806
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 82,732
IBNR	$ 5,493
Cumulative Number of Reported Claims | Claim	2,396
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 88,904
IBNR	$ 9,855
Cumulative Number of Reported Claims | Claim	2,162
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 103,688
IBNR	$ 7,681
Cumulative Number of Reported Claims | Claim	2,531
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 126,971
IBNR	$ 23,882
Cumulative Number of Reported Claims | Claim	2,933
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 132,136
IBNR	$ 53,525
Cumulative Number of Reported Claims | Claim	2,898
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 158,458
IBNR	$ 95,477
Cumulative Number of Reported Claims | Claim	3,935
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2019
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 214,653
IBNR	$ 178,172
Cumulative Number of Reported Claims | Claim	4,038