Fair Value Measurements (Quantitative Information Used as Level 3 Inputs) (Details) $ in Thousands	Mar. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	$ 1,058,714	$ 1,087,377
Other liabilities	(25,219)	(28,213)
Total other assets and (liabilities)	26,442	32,944
Fair Value, Measurements, Recurring
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	1,058,714	1,087,377
Total other assets and (liabilities)	3,078	21,668
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	17,731,279	17,283,908
Fair Value, Measurements, Recurring | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	72,620	74,634
Total other assets and (liabilities)	1,223	4,731
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	73,843	79,365
Fair Value, Measurements, Recurring | Private equity investments
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	240,277	271,047
Fair Value, Measurements, Recurring | Private equity investments | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	72,620	74,634
Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	1,822
Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other investments	70,798
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Total other assets and (liabilities)	1,223	4,731
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Total other assets and (liabilities)	1,223	$ 4,731
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Internal valuation model | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other liabilities	(1,200)
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other liabilities	(1,232)
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Other liabilities	(8,556)
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group three | Internal valuation model | Fair Value (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Total other assets and (liabilities)	$ 11,011
Manager pricing | Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Fair Value (Level 3) | Low
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	151.69
Manager pricing | Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Fair Value (Level 3) | High
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	158.90
Manager pricing | Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	151.69
Discount rate | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Fair Value (Level 3) | Low
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	0.080
Discount rate | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Fair Value (Level 3) | High
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	0.100
Discount rate | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	0.080
Discount rate | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share)	0.004
Liquidity discount | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	0.150
Liquidity discount | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share)	0.013
Bond price | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Fair Value (Level 3) | Low
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	90.99
Bond price | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Fair Value (Level 3) | High
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	98.19
Bond price | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share) | $ / shares	94.46
Net undiscounted cash flows | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share)	11,371,000
Expected loss ratio | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share)	0.305
Expected loss ratio | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group three | Internal valuation model | Fair Value (Level 3) | Weighted Average or Actual
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Significant observable and unobservable inputs (in dollars per share)	0.000