Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Other Invested Assets [Line Items]
Other invested assets	[1]	$ 333,382	$ 263,074
Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		282,946	225,661
Company’s investment in separate accounts
Other Invested Assets [Line Items]
Other invested assets		42,317	37,404
Derivatives
Other Invested Assets [Line Items]
Other invested assets		8,119	9
Equity Method Investments | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		214,898	184,063
Equity Method Investments | Private equity | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		147,660	123,957
Equity Method Investments | Hedge funds | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		58,329	53,066
Equity Method Investments | Real estate-related | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		8,909	7,040
Cost-method Investments
Other Invested Assets [Line Items]
Other invested assets			40,900
Fair Value | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		68,048	41,598
Fair Value | Private equity | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		59,230	35,686
Fair Value | Hedge funds | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		774	726
Fair Value | Real estate-related | Total LPS/LLCS
Other Invested Assets [Line Items]
Other invested assets		$ 8,044	$ 5,186

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