Fair Value Measurements (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments	$ 11,171,655,000	$ 8,088,870,000
Other investments	1,087,377,000	784,933,000
Other assets	32,944,000	4,968,000
Other liabilities	(28,213,000)	(13,327,000)
Carrying Value	1,384,105,000	991,127,000
Fair Value	1,468,618,000	974,714,000
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments	11,171,655,000	8,088,870,000
Other investments	1,087,377,000	784,933,000
Other assets	21,668,000	4,040,000
Other investments | Net investment income
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) included in net investment income	12,200,000	(8,300,000)	$ 24,700,000
Other assets and (liabilities) | Other income (loss)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) included in net investment income	0	0	$ 0
Municipal | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments	0	6,854,000
Private equity investments | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	271,047,000	242,647,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments	0	0
Other investments	74,634,000	54,545,000
Other assets	4,731,000
Other liabilities		(8,359,000)
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	74,634,000
Significant Unobservable Inputs (Level 3) | Municipal | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments		0
Significant Unobservable Inputs (Level 3) | Private equity investments | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	74,634,000	$ 54,545,000
Significant Unobservable Inputs (Level 3) | Private equity investments | External valuation model | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	10,327,000
Significant Unobservable Inputs (Level 3) | Private equity investments | Internal valuation model | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	64,307,000
Significant Unobservable Inputs (Level 3) | Assumed reinsurance contract | Internal valuation model | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	500,000
Other liabilities	(8,767,000)
Significant Unobservable Inputs (Level 3) | Assumed and ceded (re)insurance contracts | Internal valuation model | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	12,997,000
Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 231,102,000
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	1.70%
Weighted average credit quality		2.50%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	2.10%
Weighted average credit quality		3.00%
Municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average credit quality		4.80%
Non-U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	1.60%
Weighted average credit quality		2.70%
Non-U.S. government-backed corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	2.00%
Weighted average credit quality		2.80%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	3.00%
Weighted average credit quality		4.90%
Agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	2.50%
Weighted average credit quality		3.50%
Weighted average life	4 years 10 months 24 days	7 years 1 month 6 days
Non-agency prime residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	3.30%
Weighted average credit quality		4.40%
Weighted average life	4 years 9 months 18 days	4 years 8 months 12 days
Alt-A non-agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	3.80%
Weighted average credit quality		4.70%
Weighted average life	6 years 3 months 18 days	6 years 3 months 18 days
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	2.60%
Weighted average credit quality		3.60%
Weighted average life	5 years 8 months 12 days	5 years
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	3.30%
Weighted average credit quality		4.30%
Weighted average life	3 years 2 months 12 days	3 years 2 months 12 days
Short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	1.60%
Weighted average credit quality		2.10%
Private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 271,047,000	$ 242,647,000
Private equity investments | Low
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	7 years
Private equity investments | High
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	10 years
Private equity investments | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 196,413,000
Senior secured bank loan funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	22,598,000	14,482,000
Investments fair value	$ 22,600,000
Senior secured bank loan funds | Low
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	4 years
Senior secured bank loan funds | High
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	5 years
Senior secured bank loan funds | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 22,598,000
Hedge funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	12,091,000	$ 11,233,000
Hedge funds | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 12,091,000