Quarterly Results (unaudited) - Quarterly Financial Data and Operating Information (Detail) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) Settlements Backlog order $ / shares	Sep. 30, 2017 USD ($) Settlements Backlog order $ / shares	Jun. 30, 2017 USD ($) Settlements Backlog order $ / shares	Mar. 31, 2017 USD ($) Settlements Backlog order $ / shares	Dec. 31, 2016 USD ($) Settlements Backlog order $ / shares	Sep. 30, 2016 USD ($) Settlements Backlog order $ / shares	Jun. 30, 2016 USD ($) Settlements Backlog order $ / shares	Mar. 31, 2016 USD ($) Settlements Backlog order $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares
Quarterly Financial Information [Line Items]
Net income	$ 124,619	$ 162,102	$ 147,877	$ 102,923	$ 150,891	$ 117,392	$ 91,676	$ 65,303	$ 537,521	$ 425,262	$ 382,927
Diluted earnings per share (USD per share) | $ / shares	$ 28.88	$ 38.02	$ 35.19	$ 25.12	$ 37.80	$ 28.46	$ 22.01	$ 15.79	$ 126.77	$ 103.61	$ 89.99
New orders (units) | order	4,306	4,200	4,678	4,424	3,645	3,477	4,324	4,137
Settlements (units) | Settlements	4,630	4,158	3,917	3,256	4,419	3,922	3,581	3,006
Backlog, end of period (units) | Backlog	8,531	8,855	8,813	8,052	6,884	7,658	8,103	7,360
Loans closed	$ 1,229,695	$ 1,115,494	$ 1,041,613	$ 843,341	$ 1,201,164	$ 1,055,163	$ 942,407	$ 753,840
Homebuilding:
Quarterly Financial Information [Line Items]
Revenues – homebuilding operations	1,781,494	1,633,726	1,512,714	1,247,587	1,718,527	1,507,451	1,361,741	1,121,504	$ 6,175,521	$ 5,709,223	$ 5,065,200
Gross profit – homebuilding operations	343,187	325,755	294,631	221,570	305,087	265,159	235,372	195,744
Mortgage Banking:
Quarterly Financial Information [Line Items]
Mortgage banking fees	$ 34,842	$ 34,194	$ 31,778	$ 29,505	$ 34,239	$ 30,118	$ 26,442	$ 22,522	$ 130,319	$ 113,321	$ 93,808