Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Other Invested Assets [Line Items]
Other invested assets	$ 420,804	$ 377,429
Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	343,996	286,490
Company’s investment in separate accounts
Other Invested Assets [Line Items]
Other invested assets	45,038	40,126
Derivative Instruments
Other Invested Assets [Line Items]
Other invested assets	31,770	50,813
Equity Method | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	268,072	216,586
Equity Method | Private equity | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	186,657	149,164
Equity Method | Hedge funds | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	61,061	57,171
Equity Method | Real estate-related | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	20,354	10,251
Fair Value | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	75,924	69,904
Fair Value | Private equity | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	62,353	60,118
Fair Value | Hedge funds | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	690	762
Fair Value | Real estate-related | Total LPs/LLCs
Other Invested Assets [Line Items]
Other invested assets	$ 12,881	$ 9,024