Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Summary Of Significant Accounting Policies [Line Items]
Impairment recognized	$ 0	$ 0
Intangible assets	15,000,000.0	15,000,000.0	$ 15,000,000.0
Goodwill	178,800,000	178,800,000	$ 178,800,000
U.S. statutory income tax rate			21.00%
Retained earnings	2,668,000,000.0	2,410,900,000	$ 2,668,000,000.0
Total assets	13,443,700,000	12,490,500,000	13,443,700,000
Total liabilities	$ 10,241,500,000	$ 9,574,300,000	$ 10,241,500,000
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date					Jan. 01, 2018
Change in accounting principle, accounting standards update, adopted					true
Change in accounting principle, accounting standards update, immaterial effect					false
Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained earnings						$ 6,500,000
Accounting Standards Update 2018-14 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020		Dec. 31, 2020
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, immaterial effect	true		true
Accounting Standards Update 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020		Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, immaterial effect	true		true
Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020		Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, immaterial effect	true		true
Accounting Standards Update 2017-08 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2019
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, immaterial effect		false
Accounting Standards Update 2017-08 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained earnings				$ (1,500,000)
Accounting Standards Update 2016-02 and 2018-11 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Total assets				35,000,000
Total liabilities				$ 35,000,000
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2019
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, immaterial effect		false
Accounting Standards Update 2018-11 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2019
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, immaterial effect		false
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			3 years
Minimum [Member] | Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			5 years
Minimum [Member] | Real Estate And Fleet Leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			4 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			30 years
Maximum [Member] | Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			7 years
Maximum [Member] | Real Estate And Fleet Leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			6 years