Employee and Agent Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 18.2	$ 16.3	$ 54.5	$ 49.8
Interest cost	$ 29.3	$ 30.8	$ 88.0	$ 95.7
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (39.1)	$ (38.5)	$ (117.5)	$ (110.4)
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	$ (4.3)	$ (4.2)	$ (12.7)	$ (7.1)
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	$ 18.1	$ 56.6	$ 52.2
Recognized net actuarial (gain) loss, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ 23.0	$ 22.5	$ 68.9	$ 80.2
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Interest cost	$ 0.7	$ 0.9	$ 2.1	$ 2.8
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)	$ (27.0)	$ (24.9)
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	$ (0.3)	$ (0.3)	$ (0.8)	$ (0.9)
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, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ (8.6)	$ (7.7)	$ (25.7)	$ (23.0)