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)				$ (17.5)		$ 14.5
Balance at Beginning of Year 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	5.3
Other Comprehensive Income (Loss)	(386.2)				(386.2)
Net Income	559.0						559.0
Dividends to Stockholders (per common share: $0.26; $0.23; $0.52; $0.46)							(103.3)
Purchases of Treasury Stock	(200.3)	[1]							(200.3)
Balance at End of Period at Jun. 30, 2018	9,446.4		30.5	2,308.6	(276.2)		10,012.4		(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 Year at Mar. 31, 2018			30.5	2,302.4	(82.4)	(82.4)	9,777.8	9,777.8	(2,528.8)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			0.0	6.2
Other Comprehensive Income (Loss)	(193.8)				(193.8)
Net Income	285.5						285.5
Dividends to Stockholders (per common share: $0.26; $0.23; $0.52; $0.46)							(50.9)
Purchases of Treasury Stock	(100.1)	[1]							(100.1)
Balance at End of Period at Jun. 30, 2018	9,446.4		30.5	2,308.6	(276.2)		10,012.4		(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 Year 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	13.9
Other Comprehensive Income (Loss)	571.7				571.7
Net Income	562.1						562.1
Dividends to Stockholders (per common share: $0.26; $0.23; $0.52; $0.46)							(112.9)
Purchases of Treasury Stock	(200.2)	[1]							(200.2)
Balance at End of Period at Jun. 30, 2019	9,453.1		30.6	2,335.6	(242.5)		10,308.9		(2,979.5)
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption					0.0		0.0
Balance at Beginning of Year at Mar. 31, 2019			30.5	2,328.5	(502.3)	$ (502.3)	10,083.1	$ 10,083.1	(2,879.3)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			0.1	7.1
Other Comprehensive Income (Loss)	259.8				259.8
Net Income	281.2						281.2
Dividends to Stockholders (per common share: $0.26; $0.23; $0.52; $0.46)							(55.4)
Purchases of Treasury Stock	(100.2)	[1]							(100.2)
Balance at End of Period at Jun. 30, 2019	$ 9,453.1		$ 30.6	$ 2,335.6	$ (242.5)		$ 10,308.9		$ (2,979.5)

[1] ¹ Includes commissions of $0.2 million for the three months ended June 30, 2019 and a de minimis amount of commissions for the three months ended June 30, 2018. For the six months ended June 30, 2019 and 2018, these amounts included commissions of $0.2 million, respectively.