Fair Value Measurements: (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Item]
Interest rate swap liabilities - Level 2	$ 83.2		$ 77.2
Fair Value, Measurements, Recurring | Level 1 measurements:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Item]
Cash Equivalents - Level 1	50.1		0
Fair Value, Measurements, Recurring | Level 2 measurements:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Item]
Long-term debt, including current maturities	8,835.9	[1]	8,777.5	[1]
Fair Value, Measurements, Recurring | Level 2 measurements: | Other liabilities | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Item]
Interest rate swap assets - Level 2	0		0.4
Interest rate swap liabilities - Level 2	$ 83.2		$ 77.2

[1] Recognized at carrying value of $8,820.6 million and $8,651.7 million in long-term debt, including current maturities, in the accompanying consolidated balance sheets as of March 31, 2015 and December 31, 2014, respectively.