Reinsurance Balances Recoverable on Paid and Unpaid Losses - Provisions for Uncollectible Reinsurance Balances Recoverable by Rating of Reinsurer (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Gross	$ 2,527,529	$ 2,186,395
Provisions for Bad Debt	147,639	156,732
Net	$ 2,379,890	$ 2,029,663
Provisions as a % of Gross	5.80%	7.20%
Reinsurers rated A- or above
Effects of Reinsurance [Line Items]
Gross	$ 1,904,268	$ 1,612,464
Provisions for Bad Debt	43,427	51,519
Net	$ 1,860,841	$ 1,560,945
Provisions as a % of Gross	2.30%	3.20%
Secured | Reinsurers Rated Below A-
Effects of Reinsurance [Line Items]
Gross	$ 487,608	$ 430,852
Provisions for Bad Debt	0	0
Net	$ 487,608	$ 430,852
Provisions as a % of Gross	0.00%	0.00%
Unsecured | Reinsurers Rated Below A-
Effects of Reinsurance [Line Items]
Gross	$ 135,653	$ 143,079
Provisions for Bad Debt	104,212	105,213
Net	$ 31,441	$ 37,866
Provisions as a % of Gross	76.80%	73.50%