Derivative Instruments - Balance sheet offsetting (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Derivative assets:
Gross amounts recognized in balance sheet		$ 74,115	$ 136,659
Derivative Asset, Fair Value, Gross Liability	[1]	3,900	696
Net amounts in the balance sheet		70,215	135,963
Gross amounts not offset in the balance sheet - financial instruments		0	0
Gross amounts not offset in the balance sheet - cash collateral		24,000	78,580
Derivative Asset		46,215	57,383
Derivative liabilities:
Gross amounts recognized in balance sheet		49,710	31,184
Derivative Liability, Fair Value, Gross Asset	[1]	3,900	696
Net amounts in the balance sheet		45,810	30,488
Gross amounts not offset in the balance sheet - financial instruments		0	0
Gross amounts not offset in the balance sheet - cash collateral		0	0
Derivative Liability		$ 45,810	$ 30,488

[1] Other assets and other liabilities include derivative instruments. The derivative balances included in each category are reported on a gross basis by level with a netting adjustment presented separately in the Collateral and Counterparty Netting column. The fair values of the individual derivative contracts are reported gross in their respective levels based on the fair value hierarchy. For further details regarding derivative fair values and associated collateral received or paid, see Note 7, "Derivative Instruments," to the Unaudited Consolidated Financial Statements.