Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income, Net of Tax [Member]
Balance at beginning of period at Dec. 31, 2019		$ 0.6	$ 1,837.3	$ 216.0	$ (63.4)	$ 2,410.9	$ (0.9)	$ 2,410.0
Balance at beginning of period at Dec. 31, 2019									$ (1,485.2)
Net income	$ 194.1					194.1
Shares purchased at cost									(118.6)
Settlement and prepayment of accelerated share repurchases and other			34.2
Net appreciation (depreciation) on available-for-sale securities	196.3			196.3
Net amount recognized as net periodic benefit					3.5
Dividends to shareholders						(74.9)
Net shares reissued at cost under employee stock-based compensation plans									5.1
Balance at end of period at Sep. 30, 2020	3,155.0	0.6	1,871.5	412.3	(59.9)	2,529.2				$ 352.4
Balance at end of period at Sep. 30, 2020									(1,598.7)
Balance at beginning of period at Jun. 30, 2020		0.6	1,866.6	384.5	(61.0)	2,435.1		2,435.1
Balance at beginning of period at Jun. 30, 2020									(1,554.1)
Net income	118.9					118.9
Shares purchased at cost									(45.0)
Settlement and prepayment of accelerated share repurchases and other			4.9
Net appreciation (depreciation) on available-for-sale securities	27.8			27.8
Net amount recognized as net periodic benefit					1.1
Dividends to shareholders						(24.8)
Net shares reissued at cost under employee stock-based compensation plans									0.4
Balance at end of period at Sep. 30, 2020	3,155.0	0.6	1,871.5	412.3	(59.9)	2,529.2				352.4
Balance at end of period at Sep. 30, 2020									(1,598.7)
Balance at beginning of period at Dec. 31, 2020	3,202.2	0.6	1,857.4	428.1	(55.6)	2,668.0		2,668.0
Balance at beginning of period at Dec. 31, 2020	(1,696.3)								(1,696.3)
Net income	255.2					255.2
Shares purchased at cost									(147.6)
Settlement and prepayment of accelerated share repurchases and other			23.3
Net appreciation (depreciation) on available-for-sale securities	(171.3)			(171.3)
Net amount recognized as net periodic benefit					2.0
Dividends to shareholders						(76.5)
Net shares reissued at cost under employee stock-based compensation plans									15.0
Balance at end of period at Sep. 30, 2021	3,102.3	0.6	1,880.7	256.8	(53.6)	2,846.7				203.2
Balance at end of period at Sep. 30, 2021	(1,828.9)								(1,828.9)
Balance at beginning of period at Jun. 30, 2021		0.6	1,873.4	304.7	(54.2)	2,837.9		$ 2,837.9
Balance at beginning of period at Jun. 30, 2021									(1,808.4)
Net income	34.0					34.0
Shares purchased at cost									(24.0)
Settlement and prepayment of accelerated share repurchases and other			7.3
Net appreciation (depreciation) on available-for-sale securities	(47.9)			(47.9)
Net amount recognized as net periodic benefit					0.6
Dividends to shareholders						(25.2)
Net shares reissued at cost under employee stock-based compensation plans									3.5
Balance at end of period at Sep. 30, 2021	3,102.3	$ 0.6	$ 1,880.7	$ 256.8	$ (53.6)	$ 2,846.7				$ 203.2
Balance at end of period at Sep. 30, 2021	$ (1,828.9)								$ (1,828.9)