Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Derivative Asset [Abstract]
Cash Collateral Received	$ 0	$ (800,000)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	147,124,000	82,814,000
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	147,124,000	82,814,000
Financial Instruments	(142,789,000)	(80,277,000)
Cash Collateral Received	0	0
Net Amount	4,335,000	2,537,000
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	7,768,000	8,413,000
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	7,768,000	8,413,000
Financial Instruments	0	0
Cash Collateral Received	0	(810,000)
Net Amount	7,768,000	7,603,000
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	1,558,000	3,106,000
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	1,558,000	3,106,000
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 1,558,000	$ 3,106,000