Reserve for Losses and Loss Adjustment Expenses (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) partition	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Minimum period for claims resolution (years)	5 years
Number of business partitions (in partitions) | partition	200
Summary of reserve for losses and loss adjustment expenses
Balance, beginning of year	$ 2,346,526	$ 2,119,847	$ 2,119,847
Less reinsurance recoverables on unpaid losses and loss adjustment expenses	390,708	343,820	343,820
Net balance, beginning of year	1,955,818	1,776,027	1,776,027
Net losses:
Current year	170,874	170,032	765,698
Favorable development of reserves established in prior years, net	(6,042)	(10,277)	(11,783)
Total	164,832	159,755	753,915
Paid related to:
Current year	(15,876)	(17,027)	(115,133)
Prior years	(163,518)	(109,079)	(458,991)
Total paid	(179,394)	(126,106)	(574,124)
Net balance, end of period	1,941,256	1,809,676	1,955,818
Plus reinsurance recoverables on unpaid losses and loss adjustment expenses	389,792	349,319	390,708
Balance, end of period	2,331,048	$ 2,158,995	2,346,526
Specialty P&C
Net losses:
Current year	$ 5,500		$ 9,200