Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Stockholders Equity [Line Items]
Balance	$ 1,486,266,000
Balance, shares	46,523,184
Common stock issuances	11,891,000
Net income (loss)	71,879,000	70,697,000	142,330,000	134,896,000
Redemption value adjustments	658,000
Foreign currency exchange translation adj.	(1,229,000)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708,000	512,000	(9,717,000)	6,834,000
Amounts reclassified into net income	519,000	518,000	2,074,000	2,074,000
Dividends declared Common $0.405 per share	(19,202,000)
Balance	1,551,735,000		1,551,735,000
Balance, shares	46,840,289		46,840,289
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	48,153,000
Balance, shares	46,523,000
Common stock issuances	317,000
Common stock issuance, shares	317,000
Other comprehensive income (loss):
Balance	48,470,000		48,470,000
Balance, shares	46,840,000		46,840,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	851,381,000
Common stock issuances	11,574,000
Other comprehensive income (loss):
Balance	862,955,000		862,955,000
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(50,175,000)
Foreign currency exchange translation adj.	(1,229,000)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708,000
Amounts reclassified into net income	519,000
Balance	(50,177,000)		(50,177,000)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	639,164,000
Net income (loss)	71,983,000
Redemption value adjustments	658,000
Other comprehensive income (loss):
Dividends declared Common $0.405 per share	(19,202,000)
Balance	692,603,000		692,603,000
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(2,257,000)
Net income (loss)	141,000
Other comprehensive income (loss):
Balance	(2,116,000)		(2,116,000)
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	20,042,000
Net income (loss)	(245,000)
Redemption value adjustments	(658,000)
Foreign currency exchange translation adj.	(43,000)
Other comprehensive income (loss):
Balance	$ 19,096,000		$ 19,096,000