CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total		Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Effect of Change	Retained Earnings	Retained Earnings Effect of Change	Treasury Stock
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption					$ (17.5)		$ 14.5
Balance at Beginning of Period at Dec. 31, 2017			$ 30.5	$ 2,303.3	127.5	$ 110.0	9,542.2	$ 9,556.7	$ (2,428.6)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			0.0	11.0
Other Comprehensive Income (Loss)	$ (978.5)				(978.5)
Net Income (Loss)	274.3						274.3
Dividends to Stockholders (per common share: $0.285; $0.260; $0.805; $0.720)							(160.2)
Purchases of Treasury Stock	(200.3)	[1]							(200.3)
Balance at End of Period at Sep. 30, 2018	8,518.2		30.5	2,314.3	(868.5)		9,670.8		(2,628.9)
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption					0.0		0.0
Balance at Beginning of Period at Jun. 30, 2018			30.5	2,308.6	(276.2)	(276.2)	10,012.4	10,012.4	(2,628.9)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			0.0	5.7
Other Comprehensive Income (Loss)	(592.3)				(592.3)
Net Income (Loss)	(284.7)						(284.7)
Dividends to Stockholders (per common share: $0.285; $0.260; $0.805; $0.720)							(56.9)
Purchases of Treasury Stock	0.0	[1]							0.0
Balance at End of Period at Sep. 30, 2018	8,518.2		30.5	2,314.3	(868.5)		9,670.8		(2,628.9)
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption					0.0		(3.4)
Balance at Beginning of Period at Dec. 31, 2018	8,621.8		30.5	2,321.7	(814.2)	(814.2)	9,863.1	9,859.7	(2,779.3)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			0.1	20.6
Other Comprehensive Income (Loss)	661.0				661.0
Net Income (Loss)	804.1						804.1
Dividends to Stockholders (per common share: $0.285; $0.260; $0.805; $0.720)							(172.3)
Purchases of Treasury Stock	(300.3)	[1]							(300.3)
Balance at End of Period at Sep. 30, 2019	9,631.6		30.6	2,342.3	(153.2)		10,491.5		(3,079.6)
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption					0.0		0.0
Balance at Beginning of Period at Jun. 30, 2019			30.6	2,335.6	(242.5)	$ (242.5)	10,308.9	$ 10,308.9	(2,979.5)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			0.0	6.7
Other Comprehensive Income (Loss)	89.3				89.3
Net Income (Loss)	242.0						242.0
Dividends to Stockholders (per common share: $0.285; $0.260; $0.805; $0.720)							(59.4)
Purchases of Treasury Stock	(100.1)	[1]							(100.1)
Balance at End of Period at Sep. 30, 2019	$ 9,631.6		$ 30.6	$ 2,342.3	$ (153.2)		$ 10,491.5		$ (3,079.6)

[1] Includes commissions of $0.1 million and $0.3 million for the three and nine months ended September 30, 2019 and $0.2 million for the nine months ended September 30, 2018