Debt (Detail)		3 Months Ended				6 Months Ended	9 Months Ended
		Sep. 30, 2016 USD ($) instrument		Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($) instrument	Jun. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) instrument		Sep. 30, 2015 USD ($) instrument	Dec. 31, 2015 USD ($) instrument	Jun. 18, 2007
Debt Instrument [Line Items]
Gains (losses) on extinguishment of debt		$ 0			$ (900,000)		$ 1,600,000		$ (900,000)
Fair Value		3,505,200,000			2,899,200,000		3,505,200,000		2,899,200,000	$ 2,887,800,000
Debt, Long-term and Short-term, Combined Amount	[1]	$ 3,153,900,000			2,714,300,000		$ 3,153,900,000		2,714,300,000	2,707,900,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.75%					3.75%
Carrying Value		$ 498,400,000			498,100,000		$ 498,400,000		498,100,000	498,100,000
Fair Value		$ 544,200,000			534,500,000		$ 544,200,000		534,500,000	528,700,000
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		2.45%					2.45%
Carrying Value		$ 495,600,000			0		$ 495,600,000		0	0
Fair Value		$ 497,500,000			0		$ 497,500,000		0	0
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.25%					6.25%
Carrying Value		$ 395,100,000			394,900,000		$ 395,100,000		394,900,000	395,000,000
Fair Value		$ 533,300,000			489,300,000		$ 533,300,000		489,300,000	490,600,000
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.625%					6.625%
Carrying Value		$ 295,800,000			295,600,000		$ 295,800,000		295,600,000	295,700,000
Fair Value		$ 402,600,000			379,500,000		$ 402,600,000		379,500,000	376,000,000
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.35%					4.35%
Carrying Value		$ 346,400,000			346,400,000		$ 346,400,000		346,400,000	346,400,000
Fair Value		$ 403,000,000			345,700,000		$ 403,000,000		345,700,000	352,800,000
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.70%					3.70%
Carrying Value		$ 395,100,000			395,000,000		$ 395,100,000		395,000,000	395,000,000
Fair Value		$ 418,700,000			358,000,000		$ 418,700,000		358,000,000	362,000,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Repurchase of debt, face amount				$ 19,800,000	18,400,000	$ 0
Debt Instrument, Interest Rate, Stated Percentage		6.70%					6.70%				6.70%
Gains (losses) on extinguishment of debt				1,600,000					(900,000)
Carrying Value		$ 593,900,000			612,600,000		$ 593,900,000		612,600,000	612,800,000
Fair Value		572,300,000			620,500,000		572,300,000		620,500,000	612,800,000
Other debt instruments
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 133,600,000			$ 171,700,000		$ 133,600,000		$ 171,700,000	$ 164,900,000
Notes Payable, Other Payables [Member] | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		2			2		2		2	2
Debt, Long-term and Short-term, Combined Amount		$ 68,400,000			$ 93,300,000		$ 68,400,000		$ 93,300,000	$ 87,100,000
Trust Preferred Debt Securities [Member] | Other debt instruments
Debt Instrument [Line Items]
Carrying Value	[2]	$ 41,200,000			$ 41,300,000		$ 41,200,000		$ 41,300,000	$ 41,200,000
Number of debt instruments | instrument		2			2		2		2	2
Senior Notes [Member] | Other debt instruments
Debt Instrument [Line Items]
Carrying Value	[3]	$ 24,000,000			$ 24,000,000		$ 24,000,000		$ 24,000,000	$ 24,000,000
Number of debt instruments | instrument		4			4		4		4	4
Surplus Note [Member] | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		0	[4]		1		0	[4]	1	1
Debt, Long-term and Short-term, Combined Amount		$ 0			$ 13,100,000		$ 0		$ 13,100,000	$ 12,600,000
Net unrealized gains on forecasted transactions | 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)				$ 100,000					$ 200,000

[1] 1Consists of both short-term and long-term debt. See Note 4 - Debt for further discussion.

[2] 1 ARX issued junior subordinated floating rate notes to trusts established by ARX in connection with issuances of trust preferred securities by the trusts (discussed below).

[3] 2 The senior notes mature in May 2033, April 2034, December 2034, and June 2035.

[4] 3 The surplus note was debt of the subsidiary disposed of by ARX in the exchange transaction during the second quarter 2016 (see Note 1 – Basis of Presentation for further discussion).