Stockholders' Equity Issuance of Common Stock in Connection with Acquisition of CYS Investments, Inc. (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Mar. 22, 2019	Jul. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Number of shares of stock issued during period (in shares)					26,950
Aggregate cash consideration exchanged for shares of CYS common stock		$ 15.0
Common Stock [Member]
Class of Stock [Line Items]
Number of shares of stock issued during period (in shares)	18,000,000	72,600,000	24,439,436	72,616,483