Debt - Selected Information about Debt Issues (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Sep. 24, 2012	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Outstanding Principal (Par Value)		$ 1,232,399
Less current maturity of long-term debt, fair value		0
Total long-term debt, fair value		1,360,461	1,191,320
Total short-term debt, fair value		229,070
Outstanding Principal (Fair Value)		1,360,461
Total long-term debt, book value		990,865	989,686
Total short-term debt, book value		229,070	319,043
Outstanding Principal (Book Value)		1,219,935	1,308,729
Notes, Due 5/15/23 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate		7.875%
Debt instrument, maturity date		May 15, 2023
Issue Date		1993-05
Periodic Interest Payments Due		5/15 & 11/15
Outstanding Principal (Par Value)		165,612
Outstanding Principal (Fair Value)		204,489
Outstanding Principal (Book Value)		163,609	163,471
Notes, Due 8/1/13 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate		7.375%
Debt instrument, maturity date		Aug. 01, 2013
Issue Date		1993-07
Periodic Interest Payments Due		2/1 & 8/1
Outstanding Principal (Par Value)		0
Outstanding Principal (Fair Value)		0
Outstanding Principal (Book Value)		0	93,956
Senior Notes, Due 6/15/16 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate		6.375%
Debt instrument, maturity date		Jun. 15, 2016
Issue Date		2006-06
Periodic Interest Payments Due		6/15 & 12/15
Outstanding Principal (Par Value)		250,000
Outstanding Principal (Fair Value)		277,185
Outstanding Principal (Book Value)		248,753	248,300
Senior Notes, Due 6/15/19 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate		9.25%
Debt instrument, maturity date		Jun. 15, 2019
Issue Date		2009-06
Periodic Interest Payments Due		6/15 & 12/15
Outstanding Principal (Par Value)		292,647
Outstanding Principal (Fair Value)		376,089
Outstanding Principal (Book Value)		290,268	289,950
Senior Notes, Due 9/15/22 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate	3.80%	3.80%
Debt instrument, maturity date		Sep. 15, 2022
Issue Date		2012-09
Periodic Interest Payments Due	Semi-annually	3/15 & 9/15
Outstanding Principal (Par Value)	300,000	150,000
Outstanding Principal (Fair Value)		145,178
Outstanding Principal (Book Value)		147,392	147,148
Junior Subordinated Debentures Due 12/15/52 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate	5.875%	5.875%
Debt instrument, maturity date		Dec. 15, 2052
Issue Date		2012-09
Periodic Interest Payments Due	Quarterly	quarterly
Outstanding Principal (Par Value)	125,000	125,000
Outstanding Principal (Fair Value)		108,450
Outstanding Principal (Book Value)		120,843	120,817
Junior Subordinated Debentures Due 3/15/36 [Member]
Debt Instrument [Line Items]
Annual Percentage Rate		3.543%
Debt instrument, maturity date		Mar. 15, 2036
Issue Date
Periodic Interest Payments Due		quarterly
Outstanding Principal (Par Value)		20,000
Outstanding Principal (Fair Value)		20,000
Outstanding Principal (Book Value)		20,000	20,000
Total Funded Debt [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)		1,003,259
Outstanding Principal (Fair Value)		1,131,391
Outstanding Principal (Book Value)		990,865	1,083,642
Long-term Debt [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)		1,003,259
Short-term Debt [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)		229,140
Current Maturity of Long-term Debt [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)		0
Total short-term debt, fair value		0
Total short-term debt, book value		0	93,956
Commercial Paper [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)		229,140
Total short-term debt, fair value		229,070
Total short-term debt, book value		$ 229,070	$ 225,087