Insurance Liabilities - Contractholder Funding Agreements (Details) $ in Millions, € in Billions	12 Months Ended
	Dec. 31, 2015 USD ($) item	Jun. 30, 2015 USD ($)	Dec. 31, 2014 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	$ 6,863.4		$ 7,338.1
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 | 1998 Funding agreement program
Funding agreement disclosures
Maximum authorized amount of funding agreements to be issued									$ 4,000.0
Funding agreements	$ 107.5		111.2
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	201.4		505.8
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	246.0		549.2
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	1,021.0		1,137.1
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	$ 3,537.0		$ 3,284.7
Additional maximum authorized amount of funding agreements to be issued		$ 4,000.0
Amount of funding agreement authorization used at time additional amount authorized		$ 5,000.0