Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Other Invested Assets [Line Items]
Other invested assets	$ 529,040	$ 474,013
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	526,167	474,011
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	2,873	2
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	515,515	463,521
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	22,283	23,414
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	313,198	273,615
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	180,034	166,492
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	10,652	10,490
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	4,116	4,115
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	174	194
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 6,362	$ 6,181