Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,486,266
Balance, shares			46,523,184
Common stock issuances			$ 22,760
Net income (loss)	$ 4,949	$ 9,627	76,932	$ 80,410	$ 137,648	$ 134,849
Redemption value adjustments			658
Foreign currency exchange translation adj.			(1,028)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708	511	1,416	1,023	(9,520)	6,695
Amounts reclassified into net income	518	$ 519	1,037	$ 1,037	2,073	$ 2,074
Dividends declared Common $0.81 per share			(38,466)
Balance	$ 1,549,633		$ 1,549,633		$ 1,549,633
Balance, shares	47,042,327		47,042,327		47,042,327
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 48,153
Balance, shares			46,523,000
Common stock issuances			$ 519
Common stock issuance, shares			519,000
Other comprehensive income (loss):
Balance	$ 48,672		$ 48,672		$ 48,672
Balance, shares	47,042,000		47,042,000		47,042,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 851,381
Common stock issuances			22,241
Other comprehensive income (loss):
Balance	$ 873,622		873,622		$ 873,622
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,175)
Foreign currency exchange translation adj.			(1,028)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,416
Amounts reclassified into net income			1,037
Balance	(48,750)		(48,750)		(48,750)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			639,164
Net income (loss)			76,932
Redemption value adjustments			658
Other comprehensive income (loss):
Dividends declared Common $0.81 per share			(38,466)
Balance	678,288		678,288		678,288
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,257)
Net income (loss)			58
Other comprehensive income (loss):
Balance	(2,199)		(2,199)		(2,199)
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			20,042
Net income (loss)			(48)
Redemption value adjustments			(658)
Foreign currency exchange translation adj.			(35)
Other comprehensive income (loss):
Balance	$ 19,301		$ 19,301		$ 19,301