Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2011	Feb. 01, 2012	Jun. 30, 2011
Document And Entity Information [Abstract]
Document Type	10-K
Document Period End Date	Dec. 31, 2011
Amendment Flag	false
Entity Registrant Name	CYS Investments, Inc.
Entity Central Index Key	0001396446
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Public Float			$ 1,036,308,794
Entity Common Stock, Shares Outstanding		111,688,636
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY