Nature of Operations and Significant Accounting Policies - Other Various Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Counterparty	Dec. 31, 2016 USD ($) Counterparty	Dec. 31, 2015 USD ($)
Long-Term Debt
Minimum maturity period at date of issuance for debt to be classified as long-term	1 year
Reinsurance
Net ceded reinsurance recoverables related to claims received	$ 427.7	$ 412.8
Amount of net ceded reinsurance recoverables related to claims received with five largest ceded reinsurers	$ 417.4	$ 390.5
Percentage of net ceded reinsurance recoverables related to claims received with five largest ceded reinsurers (as a percent)	98.00%	95.00%
Number of largest ceded reinsurers | Counterparty	5	5
Total amount recoverable from reinsurers	$ 864.3	$ 837.1
Premiums and other considerations:
Premiums and other considerations, Direct	6,699.3	5,753.8	$ 5,710.8
Premiums and other considerations, Assumed	1.9	1.7	1.9
Premiums and other considerations, Ceded	(483.8)	(456.4)	(402.4)
Premiums and other considerations	6,217.4	5,299.1	5,310.3
Benefits, claims and settlement expenses:
Benefits, claims and settlement expenses, Direct	8,216.3	7,202.5	7,196.7
Benefits, claims and settlement expenses, Assumed	27.7	28.1	28.7
Benefits, claims and settlement expenses, Ceded	(421.4)	(317.4)	(527.7)
Benefits, claims and settlement expenses	7,822.6	6,913.2	$ 6,697.7
Separate Accounts
Separate account that primarily includes shares of Principal Financial Group, Inc. stock that were allocated and issued to eligible participants of qualified employee benefit plans as part of the 2001 demutualization	$ 170.5	$ 158.4