Investments (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) funds	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments, at fair value	$ 359,526	$ 336,856
Cash, cash equivalents, restricted cash and investments pledged as collateral	4,687,599	4,056,788
Investments held in trust	$ 4,564,394	4,007,215
Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	5 years
Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	10 years
Managed investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments, at fair value	$ 359,526	336,856
Managed investments | Recurring
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments, at fair value	359,526	336,856
Managed investments | Recurring | Subject to redemption restriction
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments, at fair value	$ 329,727	308,562
Managed investments | Recurring | Subject to redemption restriction | Investment fund A
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments number of funds | funds	1
Other investments, at fair value	$ 171,187	$ 167,910
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Redemption Restriction Period	3 years