Derivative Instruments - Balance sheet offsetting (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Derivative assets:
Gross amounts recognized in balance sheet	$ 257,880		$ 136,659
Derivative Asset, Fair Value, Gross Liability	587	[1]	696	[1]
Net amounts in the balance sheet	257,293		135,963
Gross amounts not offset in the balance sheet - financial instruments	0		0
Gross amounts not offset in the balance sheet - cash collateral	183,675		78,580
Derivative Asset	73,618		57,383
Derivative liabilities:
Gross amounts recognized in balance sheet	122,235		31,184
Derivative Liability, Fair Value, Gross Asset	587	[1]	696	[1]
Net amounts in the balance sheet	121,648		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	$ 121,648		$ 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.