Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Other Invested Assets [Line Items]
Other invested assets	[1]	$ 211,819	$ 335,811
LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		204,478	184,632
Real estate held through direct ownership
Other Invested Assets [Line Items]
Other invested assets		7,257	0
Derivatives
Other Invested Assets [Line Items]
Other invested assets		84	151,179
Equity Method | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		197,264	178,318
Equity Method | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		26,283	25,801
Equity Method | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		123,655	106,474
Equity Method | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		47,326	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		7,214	6,314
Fair Value | Private equity | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		4,324	3,500
Fair Value | Hedge funds | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		267	302
Fair Value | Real estate-related | LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets		$ 2,623	$ 2,512

[1] (1)Prior period amounts have been reclassified to conform to current period presentation. See "Adoption of ASU 2016-01" in Note 2 for details.