Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Net Amounts of Assets Presented in the Statement of Financial Position	$ 38,293	$ 89,361
Liabilities
Gross Amounts of Recognized Liabilities	3,314,414	2,480,181
Net Amounts of Liabilities Presented in the Statement of Financial	3,314,414	2,480,181
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	3,305,201	2,476,768
Cash Collateral Pledged	6,859	3,413
Net Amount	2,354
Derivatives
Assets
Gross Amounts of Recognized Assets	38,293	89,361
Net Amounts of Assets Presented in the Statement of Financial Position	38,293	89,361
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	13,677	491
Net Amount	24,616	88,870
Liabilities
Gross Amounts of Recognized Liabilities	22,890	3,904
Net Amounts of Liabilities Presented in the Statement of Financial	22,890	3,904
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	13,677	491
Cash Collateral Pledged	6,859	3,413
Net Amount	2,354
Repurchase agreements
Liabilities
Gross Amounts of Recognized Liabilities	3,291,524	2,476,277
Net Amounts of Liabilities Presented in the Statement of Financial	3,291,524	2,476,277
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	$ 3,291,524	$ 2,476,277