Resale and Repurchase Agreements Remaining Contractual Maturities of Repurchase Agreements (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 253,016	$ 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	315	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	553,331	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	288,370	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	24,095	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	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	312,465	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	336,448	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	28,917	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	200,000	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	565,365	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	7,342	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	65,000	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	72,342	257,000
Repurchase Agreements [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	885,176	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	118,327	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	500,000	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	$ 1,503,503	$ 1,956,582