Share-Based Compensation - Summary of Information Of SERP (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
SERP expense	$ 5,985	$ 4,252	$ 3,541
SERP
Defined Benefit Plan Disclosure [Line Items]
SERP expense	$ 503	$ 511	$ 538
Treasury shares issued to fund SERP expense (in shares)	9	13	15
SERP trust account balance at December 31	$ 42,467	$ 34,599	$ 37,765
Unrealized gain (loss) recorded in SERP liability account	$ 4,534	$ 1,495	$ (62)