Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative Asset [Abstract]
Cash Collateral Received	$ (5,200)	$ (1,000)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	108,044	158,147
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	108,044	158,147
Financial Instruments	0	0
Cash Collateral Received	(108,044)	(158,147)
Net Amount	0	0
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	18,739	6,787
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	18,739	6,787
Financial Instruments	0	0
Cash Collateral Received	(5,168)	(990)
Net Amount	13,571	5,797
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	11,178	22,290
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	11,178	22,290
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 11,178	$ 22,290