Derivative Instruments (Derivative Instruments Offsetting Balance Sheet) (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Derivative Assets [Abstract]
Derivative Fair Value Of Free Standing Derivative Asset		$ 198,137	$ 198,530
Gross Amounts Offset in the Balance Sheet		(16,851)	(20,258)
Net Amounts Presented in the Balance Sheet		181,286	178,272
Derivative Asset, Fair Value of Collateral	[1]	0	(862)
Gross Amounts Not Offset in the Balance Sheet - Cash Collateral Pledged/Received	[2]	(194,067)	(185,900)
Net Amount		(12,781)	(8,490)
Derivative Liabilities [Abstract]
Derivative Fair Value Of Free Standing Derivative Liability		25,922	44,499
Gross Amounts Offset in the Balance Sheet		(16,851)	(20,258)
Net Amounts Presented in the Balance Sheet		9,071	24,241
Gross Amounts Not Offset in the Balance Sheet - Financial Instruments	[3]	(57,302)	(58,156)
Gross Amounts Not Offset in the Balance Sheet - Cash Collateral Pledged/Received	[4]	(9,030)	(22,221)
Net Amount		$ (57,261)	$ (56,136)

[1] Consists of U.S. Treasury and government securities.

[2] Included in cash and cash equivalents, with obligation to return cash collateral recorded in other liabilities.

[3] Included in available-for-sale securities, primarily consists of U.S. Treasury and government agency securities.

[4] Consists of receivable from counterparty, included in other assets.