Employee and Agent Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 18.2	$ 16.7	$ 36.3	$ 33.5
Interest cost	$ 29.4	$ 32.1	$ 58.7	$ 64.9
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (39.2)	$ (36.4)	$ (78.4)	$ (71.9)
Expected return on plan assets, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Amortization of prior service (benefit) cost	$ (4.2)	$ (2.0)	$ (8.4)	$ (2.9)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Recognized net actuarial (gain) loss	$ 18.9	$ 17.2	$ 37.7	$ 34.1
Recognized net actuarial (gain) loss, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ 23.1	$ 27.6	$ 45.9	$ 57.7
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Interest cost	$ 0.7	$ 1.0	$ 1.4	$ 1.9
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (9.0)	$ (8.3)	$ (18.0)	$ (16.6)
Expected return on plan assets, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Amortization of prior service (benefit) cost	$ (0.3)	$ (0.3)	$ (0.5)	$ (0.6)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Recognized net actuarial (gain) loss, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ (8.6)	$ (7.6)	$ (17.1)	$ (15.3)