Employee and Agent Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Plan actuarial gain and loss amortization disclosures
Allowable corridor not used for amortization of actuarial gains and losses of qualified pension plan (as a percent)	10.00%
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 73.0	$ 67.1	$ 65.0
Interest cost	$ 119.5	$ 124.4	$ 134.9
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (157.0)	$ (144.2)	$ (155.0)
Expected return on plan assets, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Amortization of prior service (benefit) cost	$ (3.4)	$ (2.3)	$ (2.2)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Recognized net actuarial (gain) loss	$ 67.8	$ 68.0	$ 77.0
Recognized net actuarial (gain) loss, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ 99.9	$ 113.0	$ 119.7
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	58.3	43.9
Prior service (benefit) cost	(6.4)	(23.1)
Amortization of net gain (loss)	(67.8)	(68.0)
Amortization of prior service benefit (cost)	3.4	2.3
Total recognized in pre-tax accumulated other comprehensive (income) loss	(12.5)	(44.9)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	87.4	68.1
Estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year
Estimated net actuarial (gain) loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	67.3
Estimated prior service cost (benefit) that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	(3.7)
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	0.1	0.1	2.1
Interest cost	$ 3.5	$ 3.9	$ 6.2
Interest cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Expected return on plan assets	$ (33.6)	$ (27.5)	$ (32.6)
Expected return on plan assets, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Amortization of prior service (benefit) cost	$ (13.9)	$ (34.6)	$ (25.9)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Recognized net actuarial (gain) loss	$ (1.5)		$ 0.2
Recognized net actuarial (gain) loss, location on consolidated statements of operations	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization	pfg:OperatingExpensesIncludingDeferredAcquisitionCostsCapitalizationAndAmortization
Net periodic benefit cost (income)	$ (45.4)	$ (58.1)	$ (50.0)
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	73.4	(44.7)
Amortization of net gain (loss)	1.5
Amortization of prior service benefit (cost)	13.9	34.6
Total recognized in pre-tax accumulated other comprehensive (income) loss	88.8	(10.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	43.4	$ (68.2)
Estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year
Estimated net actuarial (gain) loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	0.1
Estimated prior service cost (benefit) that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	$ (1.2)