Insurance Liabilities - Contractholder Funding Agreements (Details) $ in Millions, € in Billions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Nov. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2006 EUR (€)	Dec. 31, 2006 USD ($)	Mar. 31, 2004 USD ($)	Dec. 31, 2001 USD ($)	Dec. 31, 1998 USD ($)
Funding agreement disclosures
Funding agreements	$ 8,106.5		$ 8,270.3
Principal Life Insurance Company
Funding agreement disclosures
Number of separate funding agreement programs | item	5
Number of funding agreement programs not guaranteed by PFG | item	3
Principal Life Insurance Company | FHLB Des Moines funding agreement program
Funding agreement disclosures
Funding agreements	$ 3,256.7		2,750.8
Principal Life Insurance Company | 1998 Funding agreement program
Funding agreement disclosures
Maximum authorized amount of funding agreements to be issued										$ 4,000.0
Funding agreements	111.1		106.7
Principal Life Insurance Company | 2006 Funding agreement program
Funding agreement disclosures
Maximum authorized amount of funding agreements to be issued						€ 4.0	$ 5,300.0
Funding agreements	177.4		702.0
Principal Life Insurance Company | 2001 Funding agreement program
Funding agreement disclosures
Maximum authorized amount of funding agreements to be issued									$ 7,000.0
Funding agreements	201.6		201.5
Principal Life Insurance Company | 2004 Funding agreement program
Funding agreement disclosures
Maximum authorized amount of funding agreements to be issued								$ 9,000.0
Funding agreements	67.8		119.8
Principal Life Insurance Company | 2011 Funding agreement program
Funding agreement disclosures
Maximum authorized amount of funding agreements to be issued					$ 5,000.0
Funding agreements	$ 4,291.9		$ 4,389.4
Additional maximum authorized amount of funding agreements to be issued		$ 4,000.0		$ 4,000.0
Amount of funding agreement authorization used at time additional amount authorized				$ 5,000.0