Shareholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Summary of Preferred and Common Share Activity

A summary of preferred and common share activity is presented in the following chart.

	Preferred Stock		Common Stock
	Issued	Treasury Stock	Issued		Treasury Stock
2015:
Balance at January 1, 2015	—	—	134,218,183		(6,287,907	)
Grants of restricted stock					6,648
Forfeitures of restricted stock					(13,950	)
Vesting of performance shares					211,287
Issuance of common stock due to exercise of stock options					1,576,485
Treasury stock acquired					(7,340,794	)
Retirement of treasury stock			(4,000,000	)	4,000,000
Balance at December 31, 2015	—	—	130,218,183		(7,848,231	)
2016:
Grants of restricted stock					12,549
Forfeitures of restricted stock					—
Vesting of performance shares					159,020
Issuance of common stock due to exercise of stock options					2,184,169
Treasury stock acquired					(6,694,582	)
Retirement of treasury stock			(3,000,000	)	3,000,000
Balance at December 31, 2016	—	—	127,218,183		(9,187,075	)
2017:
Grants of restricted stock					9,135
Vesting of performance shares					119,896
Issuance of common stock due to exercise of stock options					1,661,808
Treasury stock acquired					(5,228,868	)
Retirement of treasury stock			(3,000,000	)	3,000,000
Balance at December 31, 2017	—	—	124,218,183		(9,625,104	)

Reconciliation of Basic and Diluted Weighted Average Shares Outstanding

A reconciliation of basic and diluted weighted-average shares outstanding used in the computation of basic and diluted earnings per share is as follows:

	Year Ended December 31,
	2017	2016	2015
Basic weighted average shares outstanding	116,342,529	120,001,191	125,094,628
Weighted average dilutive options outstanding	2,640,965	2,366,594	1,662,607
Diluted weighted average shares outstanding	118,983,494	122,367,785	126,757,235