Operations and Summary of Significant Accounting Policies - Shares and EPS (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 $ / shares	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Dec. 31, 2015 $ / shares	Sep. 30, 2015 $ / shares	Jun. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Dec. 31, 2016 USD ($) vote / shares series $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 1999 rightpershare
Common Stock
Voting rights per common share | vote / shares									1
Number of rights per common stock as a part of dividend distribution | rightpershare												1
Percentage acquisition of common stock triggering exercise terms of rights, minimum									15.00%
Ownership interest held by any stockholder at date of rights distribution to which additional exercise terms for the Rights distributed apply (as a percent)									25.00%
Subsequent increase in ownership interest held by any stockholder after rights distribution to which additional exercise terms for the Rights distributed apply (as a percent)									40.00%
Redemption price per right (in cents per right) | $ / shares									$ 0.01
Preferred Stock
Number of series by which the Board may issue authorized and unissued preferred stock, minimum | series									1
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income | $									$ 89,734	$ 67,888	$ 63,531
Basic weighted-average shares outstanding									48,084	48,952	48,977
Dilutive effect of potential common stock equivalents - stock options (in shares)									211	229	217
Diluted weighted-average shares outstanding									48,295	49,181	49,194
Net earnings per share:
Basic (in USD per share) | $ / shares	$ 0.37	$ 0.62	$ 0.54	$ 0.34	$ 0.30	$ 0.44	$ 0.44	$ 0.20	$ 1.87	$ 1.39	$ 1.30
Diluted (in USD per share) | $ / shares	$ 0.36	$ 0.62	$ 0.54	$ 0.34	$ 0.30	$ 0.44	$ 0.43	$ 0.20	$ 1.86	$ 1.38	$ 1.29
Potentially dilutive securities excluded from earnings per diluted share because their effect is anti-dilutive (in shares)									0	0	0