Consolidated Statements of Preferred Stock and Common Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	AOCI Attributable to Parent	Unallocated ESSOP Shares	Convertible Preferred Stock Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 502.1	$ (502.1)
Balance at Dec. 31, 2017		$ 269.2	$ 815.2	3,206.9	474.2	$ (32.4)	$ 0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance at Dec. 31, 2017				3,708.9	(27.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.0	1.0
Net issuance of shares under stock based compensation plans		0.6	10.1
Conversion of senior debentures		32.2	438.1
Stock based compensation			2.8
ESSOP shares released			0.6			1.7
Net income (loss)	$ 4.0			4.0
Dividends on common stock				(54.9)
Foreign currency translation and other adjustments	(3.7)				(3.7)
Net unrealized gains (losses) on securities, net of tax	(117.8)				(117.8)
Net adjustment related to defined benefit pension plans, net of tax	0.6				0.6
Balance at Mar. 31, 2018		302.1	1,267.9	3,658.1	(148.9)	(30.6)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption				18.4	0.0
Balance at Dec. 31, 2018	5,146.2	302.7	1,277.6	3,849.8	(210.0)	(73.9)	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance at Dec. 31, 2018				3,868.3	(210.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.0	0.2
Net issuance of shares under stock based compensation plans		0.2	2.8
Conversion of senior debentures		0.0	0.0
Stock based compensation			2.8
ESSOP shares released			0.5			2.2
Net income (loss)	412.2			412.2
Dividends on common stock				(59.6)
Foreign currency translation and other adjustments	2.3				2.3
Net unrealized gains (losses) on securities, net of tax	138.3				138.3
Net adjustment related to defined benefit pension plans, net of tax	(1.2)				(1.2)
Balance at Mar. 31, 2019	$ 5,665.6	$ 302.9	$ 1,284.1	$ 4,220.8	$ (70.6)	$ (71.7)	$ 0.0