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	$ 75.2					75.2
Shares purchased at cost									(73.6)
Settlement and prepayment of accelerated share repurchases and other			29.3
Net appreciation (depreciation) on available-for-sale securities	168.5			168.5
Net amount recognized as net periodic benefit					2.4
Dividends to shareholders						(50.1)
Net shares reissued at cost under employee stock-based compensation plans									4.7
Balance at end of period at Jun. 30, 2020	3,071.7	0.6	1,866.6	384.5	(61.0)	2,435.1				$ 323.5
Balance at end of period at Jun. 30, 2020									(1,554.1)
Balance at beginning of period at Mar. 31, 2020		0.6	1,861.6	132.8	(62.2)	2,344.9		2,344.9
Balance at beginning of period at Mar. 31, 2020									(1,541.1)
Net income	115.2					115.2
Shares purchased at cost									(14.4)
Settlement and prepayment of accelerated share repurchases and other			5.0
Net appreciation (depreciation) on available-for-sale securities	251.7			251.7
Net amount recognized as net periodic benefit					1.2
Dividends to shareholders						(25.0)
Net shares reissued at cost under employee stock-based compensation plans									1.4
Balance at end of period at Jun. 30, 2020	3,071.7	0.6	1,866.6	384.5	(61.0)	2,435.1				323.5
Balance at end of period at Jun. 30, 2020									(1,554.1)
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	221.2					221.2
Shares purchased at cost									(123.6)
Settlement and prepayment of accelerated share repurchases and other			16.0
Net appreciation (depreciation) on available-for-sale securities	(123.4)			(123.4)
Net amount recognized as net periodic benefit					1.4
Dividends to shareholders						(51.3)
Net shares reissued at cost under employee stock-based compensation plans									11.5
Balance at end of period at Jun. 30, 2021	3,154.0	0.6	1,873.4	304.7	(54.2)	2,837.9				250.5
Balance at end of period at Jun. 30, 2021	(1,808.4)								(1,808.4)
Balance at beginning of period at Mar. 31, 2021		0.6	1,864.0	242.6	(54.9)	2,734.9		$ 2,734.9
Balance at beginning of period at Mar. 31, 2021									(1,740.4)
Net income	128.5					128.5
Shares purchased at cost									(73.3)
Settlement and prepayment of accelerated share repurchases and other			9.4
Net appreciation (depreciation) on available-for-sale securities	62.1			62.1
Net amount recognized as net periodic benefit					0.7
Dividends to shareholders						(25.5)
Net shares reissued at cost under employee stock-based compensation plans									5.3
Balance at end of period at Jun. 30, 2021	3,154.0	$ 0.6	$ 1,873.4	$ 304.7	$ (54.2)	$ 2,837.9				$ 250.5
Balance at end of period at Jun. 30, 2021	$ (1,808.4)								$ (1,808.4)