Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pre tax total accumulated other comprehensive income, beginning balance	$ 1,645.9	$ 1,338.7	$ 1,439.5	$ 1,234.5	$ 1,234.5
Investment securities, pre tax	177.2	180.9	431.7	323.6
Net non-credit related OTTI losses, adjusted for valuation changes, pre tax	0.0	0.0	0.0	(0.1)
Forecasted transactions, pre tax	(12.0)	0.0	(8.0)	0.0
Foreign currency translation adjustment, pre tax	(0.2)	(0.1)	0.3	0.6
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	(1.9)	(2.2)	(3.4)	(5.5)
Total other comprehensive income (loss) before reclassifications, pre tax	163.1	178.6	420.6	318.6
Total reclassification adjustment for amounts realized in net income, pre tax	34.9	40.3	86.0	76.1
Total other comprehensive income (loss), pre tax	128.2	138.3	334.6	242.5
Pre tax total accumulated other comprehensive income, ending balance	1,774.1	1,477.0	1,774.1	1,477.0	1,439.5
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Accumulated other comprehensive income loss tax (Provision) benefit, beginning balance	(578.4)	(470.7)	(506.1)	(434.1)	(434.1)
Investment securities, Total tax (provision) benefit	(62.1)	(63.5)	(151.3)	(113.8)
Net non-credit related OTTI losses, adjusted for valuation changes, Total tax (provision) benefit	0.0	0.0	0.0	0.1
Forecasted transactions, Total tax (provision) benefit	4.2	0.0	2.8	0.0
Foreign currency translation adjustment, Total tax (provision) benefit	0.1	0.0	(0.1)	(0.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	0.6	0.8	1.2	2.0
Total other comprehensive income (loss) before reclassifications, Total tax (provision) benefit	(57.2)	(62.7)	(147.4)	(111.9)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(12.2)	(14.2)	(30.1)	(26.8)
Total other comprehensive income (loss), Total tax (provision) benefit	(45.0)	(48.5)	(117.3)	(85.1)
Accumulated other comprehensive income loss tax (Provision) benefit, ending balance	(623.4)	(519.2)	(623.4)	(519.2)	(506.1)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,067.5	868.0	933.4	800.4	800.4
Investment securities, after tax	115.1	117.4	280.4	209.8
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	(7.8)	0.0	(5.2)	0.0
Foreign currency translation adjustment, after tax	(0.1)	(0.1)	0.2	0.4
Other comprehensive (income) loss attributable to NCI	(1.3)	(1.4)	(2.2)	(3.5)	3.2
Total other comprehensive income (loss) before reclassifications, after tax	105.9	115.9	273.2	206.7
Total reclassification adjustment for amounts realized in net income, after tax	22.7	26.1	55.9	49.3
Other comprehensive income (loss)	83.2	89.8	217.3	157.4
After tax total accumulated other comprehensive income, ending balance	1,150.7	957.8	1,150.7	957.8	933.4
Net realized gains (losses) on securities	45.9	32.5	98.8	49.9
Interest expense	43.4	34.3	80.2	68.5
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,072.0	878.5	939.6	809.0	809.0
Investment securities, after tax	115.1	117.4	280.4	209.8
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment, after tax	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) before reclassifications, after tax	115.1	117.4	280.4	209.8
Total reclassification adjustment for amounts realized in net income, after tax	22.5	25.8	55.4	48.7
Other comprehensive income (loss)	92.6	91.6	225.0	161.1
After tax total accumulated other comprehensive income, ending balance	1,164.6	970.1	1,164.6	970.1	939.6
Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(7.1)	(8.5)	(9.4)	(8.2)	(8.2)
Investment securities, after tax	0.0	0.0	0.0	0.0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	(7.8)	0.0	(5.2)	0.0
Foreign currency translation adjustment, after tax	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) before reclassifications, after tax	(7.8)	0.0	(5.2)	0.0
Total reclassification adjustment for amounts realized in net income, after tax	0.2	0.3	0.5	0.6
Other comprehensive income (loss)	(8.0)	(0.3)	(5.7)	(0.6)
After tax total accumulated other comprehensive income, ending balance	(15.1)	(8.8)	(15.1)	(8.8)	(9.4)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(0.8)	(1.0)	(1.1)	(1.5)	(1.5)
Investment securities, after tax	0.0	0.0	0.0	0.0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment, after tax	(0.1)	(0.1)	0.2	0.4
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) before reclassifications, after tax	(0.1)	(0.1)	0.2	0.4
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(0.1)	(0.1)	0.2	0.4
After tax total accumulated other comprehensive income, ending balance	(0.9)	(1.1)	(0.9)	(1.1)	(1.1)
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	3.4	(1.0)	4.3	1.1	1.1
Investment securities, after tax	0.0	0.0	0.0	0.0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment, after tax	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	(1.3)	(1.4)	(2.2)	(3.5)
Total other comprehensive income (loss) before reclassifications, after tax	(1.3)	(1.4)	(2.2)	(3.5)
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(1.3)	(1.4)	(2.2)	(3.5)
After tax total accumulated other comprehensive income, ending balance	2.1	(2.4)	2.1	(2.4)	$ 4.3
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income pretax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	(2.6)	(0.2)	(3.6)	(0.2)
Net realized gains (losses) on securities	37.2	40.0	88.8	75.3
Interest expense	0.3	0.5	0.8	1.0
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0.9	0.1	1.3	0.1
Net realized gains (losses) on securities	(13.0)	(14.1)	(31.1)	(26.5)
Interest expense	(0.1)	(0.2)	(0.3)	(0.4)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income after tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	(1.7)	(0.1)	(2.3)	(0.1)
Net realized gains (losses) on securities	24.2	25.9	57.7	48.8
Interest expense	0.2	0.3	0.5	0.6
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	(1.7)	(0.1)	(2.3)	(0.1)
Net realized gains (losses) on securities	24.2	25.9	57.7	48.8
Interest expense	0.0	0.0	0.0	0.0
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0.0	0.0	0.0	0.0
Net realized gains (losses) on securities	0.0	0.0	0.0	0.0
Interest expense	0.2	0.3	0.5	0.6
Reclassification out of Accumulated Other Comprehensive Income | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0.0	0.0	0.0	0.0
Net realized gains (losses) on securities	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Reclassification out of Accumulated Other Comprehensive Income | Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0.0	0.0	0.0	0.0
Net realized gains (losses) on securities	0.0	0.0	0.0	0.0
Interest expense	$ 0.0	$ 0.0	$ 0.0	$ 0.0