Investments - Other Investments, at Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Investments [Line Items]
Other investments, at fair value	$ 2,524,420	$ 1,957,757
Valuation lag period	3 months
Net additional subscriptions	$ 796,918	901,071	$ 109,885
Hedge funds
Other Investments [Line Items]
Other investments, at fair value	1,121,904	852,584
Fixed income funds
Other Investments [Line Items]
Other investments, at fair value	481,039	403,858
Equity funds
Other Investments [Line Items]
Other investments, at fair value	410,149	333,681
Private equity funds
Other Investments [Line Items]
Other investments, at fair value	329,885	248,628
Unfunded commitments	482,300
CLO equity funds
Other Investments [Line Items]
Other investments, at fair value	87,509	37,260
CLO equities
Other Investments [Line Items]
Other investments, at fair value	87,555	39,052
Private credit funds
Other Investments [Line Items]
Other investments, at fair value	0	33,381
Other
Other Investments [Line Items]
Other investments, at fair value	6,379	9,313
Other investments
Other Investments [Line Items]
Unrealized gains	441,636	$ (163,976)	$ 102,388
Net additional subscriptions	214,300
Less than 1 Year
Other Investments [Line Items]
Other investments, at fair value	1,379,413
Less than 1 Year | Hedge funds
Other Investments [Line Items]
Other investments, at fair value	343,005
Less than 1 Year | Fixed income funds
Other Investments [Line Items]
Other investments, at fair value	477,935
Less than 1 Year | Equity funds
Other Investments [Line Items]
Other investments, at fair value	410,149
Less than 1 Year | Private equity funds
Other Investments [Line Items]
Other investments, at fair value
Less than 1 Year | CLO equity funds
Other Investments [Line Items]
Other investments, at fair value	60,735
Less than 1 Year | CLO equities
Other Investments [Line Items]
Other investments, at fair value	87,555
Less than 1 Year | Other
Other Investments [Line Items]
Other investments, at fair value	34
1-2 years
Other Investments [Line Items]
Other investments, at fair value	624,233
1-2 years | Hedge funds
Other Investments [Line Items]
Other investments, at fair value	624,233
1-2 years | Fixed income funds
Other Investments [Line Items]
Other investments, at fair value	0
1-2 years | Equity funds
Other Investments [Line Items]
Other investments, at fair value	0
1-2 years | Private equity funds
Other Investments [Line Items]
Other investments, at fair value	0
1-2 years | CLO equity funds
Other Investments [Line Items]
Other investments, at fair value	0
1-2 years | CLO equities
Other Investments [Line Items]
Other investments, at fair value	0
1-2 years | Other
Other Investments [Line Items]
Other investments, at fair value	0
2-3 years
Other Investments [Line Items]
Other investments, at fair value	26,774
2-3 years | Hedge funds
Other Investments [Line Items]
Other investments, at fair value	0
2-3 years | Fixed income funds
Other Investments [Line Items]
Other investments, at fair value	0
2-3 years | Equity funds
Other Investments [Line Items]
Other investments, at fair value	0
2-3 years | Private equity funds
Other Investments [Line Items]
Other investments, at fair value	0
2-3 years | CLO equity funds
Other Investments [Line Items]
Other investments, at fair value	26,774
2-3 years | CLO equities
Other Investments [Line Items]
Other investments, at fair value	0
2-3 years | Other
Other Investments [Line Items]
Other investments, at fair value	0
More than 3 years
Other Investments [Line Items]
Other investments, at fair value	102,600
More than 3 years | Hedge funds
Other Investments [Line Items]
Other investments, at fair value	102,600
More than 3 years | Fixed income funds
Other Investments [Line Items]
Other investments, at fair value	0
More than 3 years | Equity funds
Other Investments [Line Items]
Other investments, at fair value	0
More than 3 years | Private equity funds
Other Investments [Line Items]
Other investments, at fair value	0
More than 3 years | CLO equity funds
Other Investments [Line Items]
Other investments, at fair value	0
More than 3 years | CLO equities
Other Investments [Line Items]
Other investments, at fair value	0
More than 3 years | Other
Other Investments [Line Items]
Other investments, at fair value	0
Not Eligible/ Restricted
Other Investments [Line Items]
Other investments, at fair value	391,400
Not Eligible/ Restricted | Hedge funds
Other Investments [Line Items]
Other investments, at fair value	52,066
Not Eligible/ Restricted | Fixed income funds
Other Investments [Line Items]
Other investments, at fair value	3,104
Not Eligible/ Restricted | Equity funds
Other Investments [Line Items]
Other investments, at fair value	0
Not Eligible/ Restricted | Private equity funds
Other Investments [Line Items]
Other investments, at fair value	329,885
Not Eligible/ Restricted | CLO equity funds
Other Investments [Line Items]
Other investments, at fair value	0
Not Eligible/ Restricted | CLO equities
Other Investments [Line Items]
Other investments, at fair value	0
Not Eligible/ Restricted | Other
Other Investments [Line Items]
Other investments, at fair value	$ 6,345