Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Shares [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]
Balance, Amount at Dec. 31, 2014	$ 1,578,172	$ 745	$ 1,479,699	$ 97,728
Balance, Shares at Dec. 31, 2014		74,510
Net income	7,508	$ 0	0	7,508
Share-based compensation, Amount	2,544	$ 1	2,543	0
Share-based compensation, Shares		75
Common share dividends	(46,073)	$ 0	0	(46,073)
Issuance of common shares, Amount	$ 8	$ 0	$ 8	$ 0
Issuance of common shares, Shares	0	0	0	0
Balance, Amount at Mar. 31, 2015	$ 1,542,159	$ 746	$ 1,482,250	$ 59,163
Balance, Shares at Mar. 31, 2015		74,585
Balance, Amount at Dec. 31, 2014	1,578,172	$ 745	1,479,699	97,728
Balance, Shares at Dec. 31, 2014		74,510
Repurchase of common shares, Amount	$ (80,800)
Repurchase of common shares, Shares	(6,200)
Balance, Amount at Dec. 31, 2015	$ 1,496,113	$ 738	1,469,722	25,653
Balance, Shares at Dec. 31, 2015		73,767
Net income	14,496	$ 0	0	14,496
Share-based compensation, Amount	1,048	$ 1	1,047	0
Share-based compensation, Shares		76
Common share dividends	(32,683)	$ 0	0	(32,683)
Repurchase of common shares, Amount	$ (64,471)	$ (52)	(64,419)	0
Repurchase of common shares, Shares	(5,200)	(5,156)
Balance, Amount at Mar. 31, 2016	$ 1,414,503	$ 687	$ 1,406,350	$ 7,466
Balance, Shares at Mar. 31, 2016		68,687