Debt - Selected Information about Debt Issues (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Outstanding Principal (Par Value)	$ 1,243,803,000
Outstanding Principal (Fair Value)	1,352,021,000
Long-term Debt	743,733,000	$ 992,130,000	[1]
Total short-term debt, book value	490,129,000	238,398,000	[1]
Debt, Long-term and Short-term, Combined Amount	$ 1,233,862,000	1,230,528,000
Notes, Due 5/15/23 [Member]
Debt Instrument [Line Items]
Annual Interest Rate	7.875%
Maturity Date	May 15, 2023
Issue Date	1993-05
Periodic Interest Payments Due	5/15 & 11/15
Outstanding Principal (Par Value)	$ 165,612,000
Outstanding Principal (Fair Value)	204,470,000
Long-term Debt	$ 163,920,000	163,758,000
Senior Notes, Due 6/15/16 [Member]
Debt Instrument [Line Items]
Annual Interest Rate	6.375%
Maturity Date	Jun. 15, 2016
Issue Date	2006-06
Periodic Interest Payments Due	6/15 & 12/15
Outstanding Principal (Par Value)	$ 250,000,000
Outstanding Principal (Fair Value)	255,354,000
Long-term Debt	$ 249,753,000	249,236,000
Senior Notes, Due 6/15/19 [Member]
Debt Instrument [Line Items]
Annual Interest Rate	9.25%
Maturity Date	Jun. 15, 2019
Issue Date	2009-06
Periodic Interest Payments Due	6/15 & 12/15
Outstanding Principal (Par Value)	$ 292,647,000
Outstanding Principal (Fair Value)	353,978,000
Long-term Debt	$ 291,002,000	290,618,000
Senior Notes, Due 9/15/22 [Member]
Debt Instrument [Line Items]
Annual Interest Rate	3.80%
Maturity Date	Sep. 15, 2022
Issue Date	2012-09
Periodic Interest Payments Due	3/15 & 9/15
Outstanding Principal (Par Value)	$ 150,000,000
Outstanding Principal (Fair Value)	148,843,000
Long-term Debt	$ 147,913,000	147,648,000
Junior Subordinated Debentures Due 12/15/52 [Member]
Debt Instrument [Line Items]
Annual Interest Rate	5.875%
Maturity Date	Dec. 15, 2052
Issue Date	2012-09
Periodic Interest Payments Due	quarterly
Outstanding Principal (Par Value)	$ 125,000,000
Outstanding Principal (Fair Value)	129,000,000
Long-term Debt	$ 120,898,000	120,870,000
Basis spread	3.30%
Junior Subordinated Debentures Due 3/15/36 [Member]
Debt Instrument [Line Items]
Annual Interest Rate	3.812%
Maturity Date	Mar. 15, 2036
Periodic Interest Payments Due	quarterly
Outstanding Principal (Par Value)	$ 20,000,000
Outstanding Principal (Fair Value)	20,000,000
Long-term Debt	20,000,000	20,000,000
Total Funded Debt [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)	1,003,259,000
Outstanding Principal (Fair Value)	1,111,645,000
Long-term Debt	993,486,000	992,130,000
Commercial Paper [Member]
Debt Instrument [Line Items]
Outstanding Principal (Par Value)	240,544,000
Total short-term debt, fair value	240,376,000
Total short-term debt, book value	$ 240,376,000	$ 238,398,000

[1] Certain balances were adjusted to give effect to discontinued operations as described in Note 1—Significant Accounting Policies.