Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unrealized gains (losses) arising during period, Pre-Tax	$ (199.3)	$ 138.7	$ (189.4)
Amount of realized gains from sales and other, Pre-tax	39.6	55.3	41.3
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	27.2	5.5	6.0
Net unrealized gains (losses), Pre-tax	(211.7)	88.9	(224.7)
Net actuarial gain (loss) arising during the period, Pre-Tax	(3.5)	(33.6)	15.0
Loss on settlement of pension obligation, Pre-Tax		12.1
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Pre-tax	(12.7)	(10.0)	(14.9)
Foreign currency translation recognized during the period, Pre-tax	(11.1)	(7.1)	(3.1)
Other comprehensive income (loss), Pre-tax	(213.6)	70.3	(197.9)
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	69.5	(45.7)	63.1
Amount of realized gains from sales and other, Tax Benefit (Expense)	0.7	0.3	(3.0)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	(9.5)	(1.9)	(2.1)
Net unrealized gains (losses), Tax Benefit (Expense)	60.7	(47.3)	58.0
Net actuarial gain (loss) arising during the period, Tax Benefit (Expense)	0.7	11.0	(4.2)
Loss on settlement of pension obligation, Tax Benefit (Expense)		(4.2)
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Tax Benefit (Expense)	(4.2)	(3.5)	(5.2)
Foreign currency translation recognized during the period, Tax Benefit (Expense)	3.9	2.5	1.1
Other comprehensive income (loss), Tax Benefit (Expense)	61.1	(41.5)	49.7
Unrealized gains (losses) arising during period, Net of Tax	(129.8)	93.0	(126.3)
Amount of realized gains from sales and other, Net of Tax	38.9	55.0	44.3
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	(17.7)	(3.6)	(3.9)
Total available-for-sale securities and derivative instruments	(151.0)	41.6	(166.7)
Net actuarial gain (loss) arising in the period, Net of Tax	(2.8)	(22.6)	10.8
Loss on settlement of pension obligation, Net of Tax		7.9
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Net of Tax	8.5	6.5	9.7
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Total	(7.2)	(4.6)	(2.0)
Total other comprehensive (loss) income, net of tax	(152.5)	28.8	(148.2)
Net of pre-tax, ceded unrealized gains (losses)		0.8	(1.2)
The Hanover Insurance Group [Member]
Total other comprehensive (loss) income, net of tax	$ (152.5)	$ 28.8	$ (148.2)