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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

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The remaining number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

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No definition available.

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Total cost of shares repurchased divided by the total number of shares repurchased.

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Number of shares that have been repurchased during the period and are being held in treasury.

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Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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Payments for Repurchase of Common Stock, Cash On Hand

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Stock Repurchase Program, Number of Additional Shares Authorized to be Repurchased

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Treasury Stock Acquired, Average Cost Per Share Including Fees And Expenses

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Equity Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended						9 Months Ended		70 Months Ended
	Oct. 27, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)								844,671	368,468
Share repurchase price (in $ per share)								$ 83.94	$ 122.09
Total cost of treasury shares acquired								$ 72,200	$ 45,000
Common stock available to repurchase (in shares)		1,197,701						1,197,701		1,197,701
Cash dividends declared per share (in $ per share)		$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.35	$ 1.35
Total cash dividends								$ 11,003	$ 12,136
Repurchase Plan, Tender Offer [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)			556,516
Share repurchase price (in $ per share)			$ 82.50
Total cost of treasury shares acquired			$ 47,200
Share repurchase price, including fees and expenses (in $ per share)			$ 84.81
Repurchase Plan, Excluding Tender Offer [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)										1,502,299
Share repurchase price (in $ per share)										$ 121.58
Total cost of treasury shares acquired										$ 182,700
Bank of Montreal Holdings [Member]
Equity, Class of Treasury Stock [Line Items]
Ownership percentage by noncontrolling owners		22.70%						22.70%		22.70%
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock available to repurchase (in shares)	200,000
Subsequent Event [Member] \| Stock Purchase Agreement [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)	1,727,746
Share repurchase price (in $ per share)	$ 93.50
Total cost of treasury shares acquired	$ 161,500
Repurchases of common shares utilizing cash on hand	131,500
Proceeds from credit facility	$ 30,000
Additional shares authorized for repurchase (in shares)	730,045