Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Long-term debt measured at fair value

The carrying amounts and estimated fair values of the Company's long-term debt, including current maturities, at December 31, 2016 and December 31, 2015, were as follows:

	As of December 31,
	2016				2015
	Carrying Value		Estimated Fair Value		Carrying Value		Estimated Fair Value		Fair value level hierarchy
	(in millions)
Senior long-term debt	$	451.9	$	463.9	$	484.2	$	477.8	Level 3
Junior long-term debt	47.1		48.1		54.3		54.6		Level 3
Class A enhanced equipment trust certificates	409.8		416.0		95.8		94.8		Level 2
Class B enhanced equipment trust certificates	103.6		105.7		25.0		25.2		Level 2
Total long-term debt	$	1,012.4	$	1,033.7	$	659.3	$	652.4

Assets and liabilities measured at fair value on a recurring basis

Assets and liabilities measured at gross fair value on a recurring basis are summarized below:

	Fair Value Measurements as of December 31, 2016
	Total		Level 1		Level 2		Level 3
	(in millions)
Cash and cash equivalents	$	700.9	$	700.9	$	—	$	—
Short-term investment securities	100.2		100.2		—		—
Total assets	$	801.1	$	801.1	$	—	$	—
Total liabilities	$	—	$	—	$	—	$	—

	Fair Value Measurements as of December 31, 2015
	Total		Level 1		Level 2		Level 3
	(in millions)
Cash and cash equivalents	$	803.6	$	803.6	$	—	$	—
Total assets	$	803.6	$	803.6	$	—	$	—
Total liabilities	$	—	$	—	$	—	$	—