Resale and Repurchase Agreements Remaining Contractual Maturities of Repurchase Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
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	$ 324,533	$ 271,820
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	101,720	1,731
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	250,000	450,000
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	676,253	723,551
Government-Sponsored Enterprise Obligations [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	31,753	149,111
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	0	0
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	31,753	149,111
Agency Mortgage-Backed Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	541,023	737,975
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	22,950	9,750
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	207,250	200,000
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	771,223	947,725
Non Agency Mortgage Backed Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	173,495	0
Non 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	0	0
Non 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	0	0
Non 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	173,495	0
Asset-Backed Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	217,719	89,601
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	85,000	30,000
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	302,719	119,601
Other Debt Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	97,734	14,780
Other Debt Securities [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
Other Debt 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
Other Debt Securities [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	97,734	14,780
Repurchase Agreements [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	1,386,257	1,263,287
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	209,670	41,481
Repurchase Agreements [Member] | Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	457,250	650,000
Repurchase Agreements [Member] | Total Repurchase Agreements [Member] [Domain]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 2,053,177	$ 1,954,768