Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 03, 2016
Statement of Stockholders' Equity [Abstract]
Common shares repurchased (in shares)									530,882	756,252	847,111
Repurchase of common shares, weighted average price (in dollars per share)									$ 59.83	$ 59.76	$ 55.64
Stock dividend declared, percent	15.00%								15.00%		10.00%	10.00%
Cash dividend declared - common shares, per share (in dollars per share)	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.24	$ 0.24	$ 0.23	$ 0.23	$ 1.08	$ 0.94	$ 0.75