Derivatives - Schedule of gross and net amounts of derivative instruments that are subject to enforceable master netting arrangements or similar agreements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	$ 28,134	$ 39,045
Financial Instruments	5,686	7,917
Cash Collateral Received	6,006	12,094
Net Amount	16,442	19,034
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	7,569	8,819
Financial Instruments	5,686	7,917
Cash Collateral Pledged	1,883	902
Net Amount	0	0
Counterparty 1
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	881	1,128
Financial Instruments	881	1,041
Cash Collateral Received	0	0
Net Amount	0	87
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	1,292	1,041
Financial Instruments	881	1,041
Cash Collateral Pledged	411	0
Net Amount	0	0
Counterparty 2
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	3,396	4,998
Financial Instruments	188	400
Cash Collateral Received	0	1,629
Net Amount	3,208	2,969
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	188	400
Financial Instruments	188	400
Cash Collateral Pledged	0	0
Net Amount	0	0
Counterparty 3
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	12,715	16,066
Financial Instruments	2,457	3,509
Cash Collateral Received	0	0
Net Amount	10,258	12,557
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	2,457	3,509
Financial Instruments	2,457	3,509
Cash Collateral Pledged	0	0
Net Amount	0	0
Counterparty 4
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	378	1,351
Financial Instruments	378	1,351
Cash Collateral Received	0	0
Net Amount	0	0
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	1,636	1,360
Financial Instruments	378	1,351
Cash Collateral Pledged	1,258	9
Net Amount	0	0
Counterparty 5
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	1,498	3,198
Financial Instruments	915	1,054
Cash Collateral Received	0	0
Net Amount	583	2,144
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	915	1,054
Financial Instruments	915	1,054
Cash Collateral Pledged	0	0
Net Amount	0	0
Counterparty 6
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	8,505	12,234
Financial Instruments	432	492
Cash Collateral Received	6,006	10,465
Net Amount	2,067	1,277
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	432	492
Financial Instruments	432	492
Cash Collateral Pledged	0	0
Net Amount	0	0
Counterparty 7
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	52	2
Financial Instruments	52	2
Cash Collateral Received	0	0
Net Amount	0	0
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	266	59
Financial Instruments	52	2
Cash Collateral Pledged	214	57
Net Amount	0	0
Counterparty 8
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet	709	0
Financial Instruments	383	0
Cash Collateral Received	0	0
Net Amount	326	0
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet	383	0
Financial Instruments	383	0
Cash Collateral Pledged	0	0
Net Amount	0	0
Counterparty 9
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Assets Presented in the condensed consolidated balance sheet		68
Financial Instruments		68
Cash Collateral Received		0
Net Amount		0
Gross Amounts not offset in the condensed consolidated balance sheet
Gross Amounts of Liabilities Presented in the condensed consolidated balance sheet		904
Financial Instruments		68
Cash Collateral Pledged		836
Net Amount		$ 0