Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Invested Assets [Line Items]
Other invested assets	$ 348,541	$ 335,811
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	305,563	184,632
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	42,978	151,179
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	297,596	178,318
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	23,844	25,801
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	179,014	106,474
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	94,738	46,043
Cost Method
Other Invested Assets [Line Items]
Other invested assets		6,000
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	7,967	6,314
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	4,142	3,500
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	263	302
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 3,562	$ 2,512