Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Other Invested Assets [Line Items]
Other invested assets	$ 600,610	$ 474,013
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	595,841	474,011
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	4,769	2
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	585,363	463,521
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	22,719	23,414
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	333,340	273,615
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	229,304	166,492
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	10,478	10,490
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	4,095	4,115
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	172	194
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 6,211	$ 6,181