Premiums and Reinsurance Related Information - Premiums and Reinsurance, Allowance For Credit Loss (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	$ 22,106	$ 19,823
Cumulative effect adjustment resulting from changes in accounting principles	23,033	23,033
Provision for expected credit losses	927	1,940
Allowance for expected credit losses at September 30, 2020	23,033	23,033
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	7,175	690
Cumulative effect adjustment resulting from changes in accounting principles	7,175	7,741
Reinsurance Recoverable, Credit Loss Expense (Reversal)	566	1,124
Allowance for expected credit losses at September 30, 2020	$ 7,741	7,741
Cumulative effect adjustment resulting from changes in accounting principles
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020		1,270
Cumulative effect adjustment resulting from changes in accounting principles		1,270
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020		5,927
Cumulative effect adjustment resulting from changes in accounting principles		$ 5,927