Investments (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) funds	Dec. 31, 2014 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities lending - Percent of market value required in collateral	102.00%
Cash, cash equivalents and investments pledged as collateral	$ 4,056,788	$ 3,150,295
Investments held in trust	$ 4,007,215	$ 3,122,074
Subject to redemption restriction
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments Number Of Funds | funds	1