Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Other Invested Assets [Line Items]
Other invested assets	$ 505,569	$ 474,013
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	488,207	474,011
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	17,362	2
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	476,808	463,521
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	22,462	23,414
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	272,972	273,615
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	181,374	166,492
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	11,399	10,490
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	4,807	4,115
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	190	194
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 6,402	$ 6,181