Employee and Agent Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 18.1	$ 16.8
Interest cost	$ 29.3	$ 32.8
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (39.2)	$ (35.5)
Expected return on plan assets, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Amortization of prior service (benefit) cost	$ (4.2)	$ (0.9)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Recognized net actuarial (gain) loss	$ 18.8	$ 16.9
Recognized net actuarial (gain) loss, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ 22.8	$ 30.1
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Interest cost	$ 0.7	$ 0.9
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (9.0)	$ (8.3)
Expected return on plan assets, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Amortization of prior service (benefit) cost	$ (0.2)	$ (0.3)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ (8.5)	$ (7.7)