Reinsurance Balances Recoverable on Paid and Unpaid Losses - Top Ten Reinsurers (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) reinsurer	Dec. 31, 2018 USD ($) reinsurer
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 2,379,890	$ 2,029,663
Reinsurer concentration risk | Top Ten Reinsurers, Rated A Minus Or Higher
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 1,292,207	$ 1,096,272
Number of external reinsurers | reinsurer	8	7
Reinsurer concentration risk | Top Ten Reinsurers, Non-Rated Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 272,125	$ 316,238
Number of external reinsurers | reinsurer	2	3
Reinsurer concentration risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 1,564,332	$ 1,412,510
Reinsurance balances recoverable | Reinsurer concentration risk
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 2,379,890	$ 2,029,663
% of Total	100.00%	100.00%
Reinsurance balances recoverable | Reinsurer concentration risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 1,564,332	$ 1,412,510
% of Total	65.70%	69.60%
Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 793,268	$ 596,312
% of Total	33.40%	29.40%
Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 22,290	$ 20,841
% of Total	0.90%	1.00%
Non-life Run-off
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 1,724,989	$ 1,504,388
Non-life Run-off | Reinsurance balances recoverable | Reinsurer concentration risk
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	1,724,989	1,504,388
Non-life Run-off | Reinsurance balances recoverable | Reinsurer concentration risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	1,154,110	1,124,079
Non-life Run-off | Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	551,636	364,098
Non-life Run-off | Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	19,243	16,211
Atrium
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	30,357	38,512
Atrium | Reinsurance balances recoverable | Reinsurer concentration risk
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	30,357	38,512
Atrium | Reinsurance balances recoverable | Reinsurer concentration risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	22,051	25,239
Atrium | Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	7,761	12,091
Atrium | Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	545	1,182
StarStone
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	624,544	486,763
StarStone | Reinsurance balances recoverable | Reinsurer concentration risk
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	624,544	486,763
StarStone | Reinsurance balances recoverable | Reinsurer concentration risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	388,171	263,192
StarStone | Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	233,871	220,123
StarStone | Reinsurance balances recoverable | Reinsurer concentration risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	2,502	3,448
Equity method investee | Reinsurer concentration risk | Hannover Ruck SE
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	261,295	279,723
Equity method investee | Reinsurer concentration risk | Lloyd's Syndicate
Ceded Credit Risk [Line Items]
Reinsurance balances recoverable on paid and unpaid losses	$ 411,030	$ 334,509