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Debt (Tables)
 3 Months Ended
Mar. 31, 2021
Debt Disclosure [Abstract]  
Debt Table Debt at each of the balance sheet periods consisted of:
 March 31, 2021March 31, 2020December 31, 2020
(millions)Carrying
Value		Fair
Value		Carrying
Value		Fair
Value		Carrying
Value		Fair
Value
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
$499.9 $506.7 $499.5 $510.7 $499.8 $510.9 
2.45% Senior Notes due 2027 (issued: $500.0, August 2016)
497.4 526.8 497.0 488.2 497.3 541.1 
6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999)
297.0 386.1 296.7 395.3 296.9 409.4 
4.00% Senior Notes due 2029 (issued: $550.0, October 2018)
545.6 623.7 545.1 605.7 545.5 660.4 
3.20% Senior Notes due 2030 (issued: $500.0, March 2020)
496.2 536.7 495.8 543.2 496.1 575.5 
6.25% Senior Notes due 2032 (issued: $400.0, November 2002)
396.0 531.4 395.8 539.6 396.0 582.0 
4.35% Senior Notes due 2044 (issued: $350.0, April 2014)
346.8 413.9 346.7 391.7 346.7 459.7 
3.70% Senior Notes due 2045 (issued: $400.0, January 2015)
395.6 430.2 395.4 406.2 395.5 481.0 
4.125% Senior Notes due 2047 (issued: $850.0, April 2017)
841.7 977.1 841.6 991.9 841.7 1,113.1 
4.20% Senior Notes due 2048 (issued: $600.0, March 2018)
590.0 698.7 589.9 675.8 590.0 806.7 
3.95% Senior Notes due 2050 (issued: $500.0, March 2020)
490.6 563.9 490.5 560.3 490.6 653.7 
Total$5,396.8 $6,195.2 $5,394.0 $6,108.6 $5,396.1 $6,793.5