Fair Value Measurements (Quantitative Information Used As Level 3 Inputs) (Details) $ in Thousands	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 1,087,377	$ 784,933
Other liabilities	(28,213)	(13,327)
Other assets and (liabilities)	32,944	4,968
Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	1,087,377	784,933
Other assets and (liabilities)	21,668	4,040
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	17,283,908	11,774,615
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	74,634	54,545
Other liabilities		(8,359)
Other assets and (liabilities)	4,731
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	79,365	46,186
Fair Value, Measurements, Recurring | Internal valuation model | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	74,634
Fair Value, Measurements, Recurring | Private equity investments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	271,047	242,647
Fair Value, Measurements, Recurring | Private equity investments | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	74,634	54,545
Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	10,327
Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	64,307
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other liabilities		(8,359)
Other assets and (liabilities)	4,731
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other liabilities		$ (8,359)
Other assets and (liabilities)	4,731
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other assets and (liabilities)	501
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other liabilities	(8,767)
Other assets and (liabilities)	500
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group three | Internal valuation model | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other assets and (liabilities)	$ 12,997
Manager pricing | Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Significant Unobservable Inputs (Level 3) | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	103.20
Manager pricing | Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Significant Unobservable Inputs (Level 3) | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	103.77
Manager pricing | Fair Value, Measurements, Recurring | Private equity investments | External valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	103.20
Discount rate | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Significant Unobservable Inputs (Level 3) | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	0.080
Discount rate | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Significant Unobservable Inputs (Level 3) | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	0.100
Discount rate | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	0.090
Discount rate | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs	0.017
Liquidity discount | Fair Value, Measurements, Recurring | Private equity investments | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	0.150
Liquidity discount | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs	0.013
Bond price | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Significant Unobservable Inputs (Level 3) | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	100.29
Bond price | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Significant Unobservable Inputs (Level 3) | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	106.26
Bond price | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group one | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs | $ / shares	103.58
Net undiscounted cash flows | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs	(11,179,000)
Expected loss ratio | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs	0.331
Expected loss ratio | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group three | Internal valuation model | Significant Unobservable Inputs (Level 3) | Weighted Average or Actual
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant observable and unobservable inputs	0.000