Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivatives
Gross Amounts of Recognized Financial Instruments	$ 82,860	$ 300,655
Securities purchased under agreement to resell
Gross Amounts of Recognized Financial Instruments	24,000	156,064
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	24,000	156,064
Financial Instruments/Collateral	(24,000)	(156,064)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	106,852	453,435
Gross Amounts Offset in the Consolidated Statement of Financial Position	(14,711)	(230,554)
Net Amounts Presented in the Consolidated Statement of Financial Position	92,141	222,881
Financial Instruments/Collateral	(92,141)	(171,221)
Net Amount	0	51,660
Derivatives
Gross Amounts of Recognized Financial Instruments	14,847	32,849
Securities sold under agreement to repurchase
Gross Amounts of Recognized Financial Instruments	0	0
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	14,847	32,849
Gross Amounts Offset in the Consolidated Statement of Financial Position	(14,847)	(32,849)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Counterparty
Derivatives
Gross Amounts of Recognized Financial Instruments	82,852	297,371
Gross Amounts Offset in the Consolidated Statement of Financial Position	(14,711)	(230,554)
Net Amounts Presented in the Consolidated Statement of Financial Position	68,141	66,817
Financial Instruments/Collateral	(68,141)	(15,157)
Net Amount	0	51,660
Derivatives
Gross Amounts of Recognized Financial Instruments	14,847	32,849
Gross Amounts Offset in the Consolidated Statement of Financial Position	(14,847)	(32,849)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0