Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other Invested Assets [Line Items]
Other invested assets	$ 474,013	$ 348,541
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	474,011	305,563
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	2	42,978
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	463,521	297,596
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	23,414	23,844
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	273,615	179,014
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	166,492	94,738
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	10,490	7,967
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	4,115	4,142
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	194	263
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 6,181	$ 3,562