Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Derivatives
Gross Amounts of Recognized Financial Instruments	$ 86,623	$ 300,655
Securities purchased under agreement to resell
Gross Amounts of Recognized Financial Instruments	106,000	156,064
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	106,000	156,064
Financial Instruments/Collateral	(106,000)	(156,064)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	192,619	453,435
Gross Amounts Offset in the Consolidated Statement of Financial Position	(10,295)	(230,554)
Net Amounts Presented in the Consolidated Statement of Financial Position	182,324	222,881
Financial Instruments/Collateral	(182,324)	(171,221)
Net Amount	0	51,660
Derivatives
Gross Amounts of Recognized Financial Instruments	10,405	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	10,405	32,849
Gross Amounts Offset in the Consolidated Statement of Financial Position	(10,405)	(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	86,619	297,371
Gross Amounts Offset in the Consolidated Statement of Financial Position	(10,295)	(230,554)
Net Amounts Presented in the Consolidated Statement of Financial Position	76,324	66,817
Financial Instruments/Collateral	(76,324)	(15,157)
Net Amount	0	51,660
Derivatives
Gross Amounts of Recognized Financial Instruments	10,405	32,849
Gross Amounts Offset in the Consolidated Statement of Financial Position	(10,405)	(32,849)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0