Resale and Repurchase Agreements Remaining Contractual Maturities of Repurchase Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
U.S. Government And Federal Agency Obligations [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 228,435	$ 210,346
U.S. Government And Federal Agency Obligations [Member] | Maturity up to 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	0	0
U.S. Government And Federal Agency Obligations [Member] | Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	300,000	300,000
U.S. Government And Federal Agency Obligations [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	528,435	510,346
Government-Sponsored Enterprise Obligations [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	185,224	356,970
Government-Sponsored Enterprise Obligations [Member] | Maturity up to 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	0	0
Government-Sponsored Enterprise Obligations [Member] | Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	2,728	24,096
Government-Sponsored Enterprise Obligations [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	187,952	381,066
Agency Mortgage-Backed Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	367,145	579,974
Agency Mortgage-Backed Securities [Member] | Maturity up to 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	2,908	2,292
Agency Mortgage-Backed Securities [Member] | Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	252,982	225,904
Agency Mortgage-Backed Securities [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	623,035	808,170
Asset-Backed Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	306,829	212,000
Asset-Backed Securities [Member] | Maturity up to 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	100,000	45,000
Asset-Backed Securities [Member] | Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	0	0
Asset-Backed Securities [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	406,829	257,000
Repurchase Agreements [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	1,087,633	1,359,290
Repurchase Agreements [Member] | Maturity up to 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	102,908	47,292
Repurchase Agreements [Member] | Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	555,710	550,000
Repurchase Agreements [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 1,746,251	$ 1,956,582