Retroactive Insurance Contracts (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Net premiums earned	$ 214,446	$ 215,788	$ 209,149	$ 208,149	$ 202,033	$ 206,070	$ 223,591	$ 187,159	$ 847,532	$ 818,853	$ 738,531
Net losses and loss adjustment expenses	233,680	$ 177,555	$ 184,435	$ 170,032	179,056	$ 169,154	184,543	$ 152,572	765,698	685,326	603,518
Gross liability for losses and loss adjustment expenses	$ 2,346,526				$ 2,119,847				2,346,526	2,119,847	$ 2,048,381	$ 1,993,428
Prospective coverage for retroactive insurance contract
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Net premiums earned									1,800	7,900
Retroactive coverage for retroactive insurance contract
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Net premiums earned									900	18,700
Retroactive insurance contract
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Net premiums earned									2,700	26,600
Net losses and loss adjustment expenses									$ 2,100	$ 25,400
Gross liability for losses and loss adjustment expenses							$ 600