Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Other Invested Assets [Line Items]
Other invested assets	$ 373,782	$ 348,541
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	373,780	305,563
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	2	42,978
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	364,039	297,596
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	24,043	23,844
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	204,961	179,014
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	135,035	94,738
Fair Value | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	9,741	7,967
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	3,854	4,142
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	268	263
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 5,619	$ 3,562