Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Other Invested Assets [Line Items]
Other invested assets	$ 919,415	$ 818,810
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	613,865	624,559
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	305,550	194,251
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	603,425	613,947
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	25,844	28,955
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	330,572	340,951
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	247,009	244,041
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	10,440	10,612
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	3,672	4,220
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	152	172
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 6,616	$ 6,220