Borrowings under Repurchase Agreements Schedule of Repurchase Agreements by Maturity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Repurchase Agreements [Line Items]
Repurchase agreements	$ 1,296,272	$ 1,481,561
Debt, Weighted Average Interest Rate	2.00%	2.56%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	39 days	44 days
Maturity up to 30 days [Member]
Repurchase Agreements [Line Items]
Repurchase agreements	$ 513,092	$ 512,505
Debt, Weighted Average Interest Rate	2.10%	2.45%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	15 days	16 days
Maturity 31 to 60 days [Member]
Repurchase Agreements [Line Items]
Repurchase agreements	$ 549,541	$ 594,199
Debt, Weighted Average Interest Rate	1.91%	2.56%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	45 days	46 days
Maturity 61 to 90 days [Member]
Repurchase Agreements [Line Items]
Repurchase agreements	$ 233,639	$ 359,861
Debt, Weighted Average Interest Rate	1.97%	2.71%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	74 days	75 days
Maturity 91 to 120 days [Member]
Repurchase Agreements [Line Items]
Repurchase agreements	$ 0	$ 0
Debt, Weighted Average Interest Rate	0.00%	0.00%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	0 days	0 days
Maturity 121 to 150 days [Member] [Member]
Repurchase Agreements [Line Items]
Repurchase agreements	$ 0	$ 0
Debt, Weighted Average Interest Rate	0.00%	0.00%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	0 days	0 days
Maturity 151 to 180 days [Member] [Member]
Repurchase Agreements [Line Items]
Repurchase agreements	$ 0	$ 14,996
Debt, Weighted Average Interest Rate	0.00%	2.72%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements	0 days	177 days