Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Debt and Credit Arrangements (Schedule of Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 19, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total principal debt	$ 786.9		$ 662.7
Unamortized debt issuance costs	(6.1)		(9.3)
Total	780.8		653.4
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Total principal debt	375.0		375.0
Senior debentures maturing September 1, 2030 [Member]
Debt Instrument [Line Items]
Total principal debt	300.0
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt	61.8		62.6
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt	$ 50.1		50.1
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Total principal debt			$ 175.0
Unamortized debt issuance costs		$ (6.1)