Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment [Member]	Treasury Stock	AOCI Attributable to Parent	AOCI Attributable to Parent Cumulative Effect, Period of Adoption, Adjustment [Member]
Balance at Dec. 31, 2018	$ 226.0	$ 0.6	$ 547.4	$ 829.8	$ (0.1)	$ (692.2)	$ (459.6)	$ 0.1
Balance at Dec. 31, 2018		48,808,239
Balance at Dec. 31, 2018						12,745,485
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201712Member			us-gaap:AccountingStandardsUpdate201712Member
Stock issuance, net		617,984
Cash dividends - per common share	(26.2)			$ (26.2)
Share-based employee compensation	2.0		2.0
Net earnings (loss)	166.8			166.8
Other comprehensive income (loss)	79.9						$ 79.9
Acquisition of treasury stock	(81.2)					$ (81.2)
Acquisition of treasury stock, shares		(994,995)				994,995
Balance at Sep. 30, 2019	367.3	$ 0.6	549.4	970.3		$ (773.4)	(379.6)
Balance at Sep. 30, 2019		48,431,228
Balance at Sep. 30, 2019						13,740,480
Balance at Jun. 30, 2019	243.1	$ 0.6	538.1	905.7		$ (740.3)	(461.0)
Balance at Jun. 30, 2019		48,661,946
Balance at Jun. 30, 2019						13,393,176
Stock issuance, net		116,586
Cash dividends - per common share	(8.6)			(8.6)
Share-based employee compensation	11.3		11.3
Net earnings (loss)	73.2			73.2
Other comprehensive income (loss)	81.4						81.4
Acquisition of treasury stock	(33.1)					$ (33.1)
Acquisition of treasury stock, shares		(347,304)				347,304
Balance at Sep. 30, 2019	367.3	$ 0.6	549.4	970.3		$ (773.4)	(379.6)
Balance at Sep. 30, 2019		48,431,228
Balance at Sep. 30, 2019						13,740,480
Balance at Dec. 31, 2019	$ 364.9	$ 0.6	555.7	1,008.2		$ (823.5)	(376.1)
Balance at Dec. 31, 2019	47,992,348	47,992,348
Balance at Dec. 31, 2019						14,271,047
Stock issuance, net		249,573
Cash dividends - per common share	$ (29.2)			(29.2)
Share-based employee compensation	(3.1)		(3.1)
Net earnings (loss)	(121.9)			(121.9)
Other comprehensive income (loss)	266.3						266.3
Acquisition of treasury stock	(34.4)					$ (34.4)
Acquisition of treasury stock, shares		(382,170)				382,170
Balance at Sep. 30, 2020	$ 442.6	$ 0.6	552.6	857.1		$ (857.9)	(109.8)
Balance at Sep. 30, 2020	47,859,751	47,859,751
Balance at Sep. 30, 2020						14,653,217
Balance at Jun. 30, 2020	$ 402.9	$ 0.6	551.7	812.8		$ (857.9)	(104.3)
Balance at Jun. 30, 2020		47,856,179
Balance at Jun. 30, 2020						14,653,217
Stock issuance, net		3,572
Cash dividends - per common share	(9.7)			(9.7)
Share-based employee compensation	0.9		0.9
Net earnings (loss)	54.0			54.0
Other comprehensive income (loss)	(5.5)						(5.5)
Balance at Sep. 30, 2020	$ 442.6	$ 0.6	$ 552.6	$ 857.1		$ (857.9)	$ (109.8)
Balance at Sep. 30, 2020	47,859,751	47,859,751
Balance at Sep. 30, 2020						14,653,217